Update documentation for FatJar generation in ktor.io

### Ktor Version
1.1.3 with Tomcat and Gradle

### Operating System
Operating system: MacOS High Sierra

### Feedback
By following the steps in [documentation](https://ktor.io/servers/deploy/packing/fatjar.html) to generate a FatJar, whenever I try to execute my Ktor application by using the command `java -jar pathToFatJar`, I receive de following error:

```
Exception in thread "main" java.lang.UnsupportedOperationException: Packages and file facades are not yet supported in Kotlin reflection. Meanwhile please use Java reflection to inspect this class: class com.tidelevel.ApplicationKt
at kotlin.reflect.jvm.internal.KClassImpl.reportUnresolvedClass(KClassImpl.kt:301)
at kotlin.reflect.jvm.internal.KClassImpl.access$reportUnresolvedClass(KClassImpl.kt:43)
at kotlin.reflect.jvm.internal.KClassImpl$Data$descriptor$2.invoke(KClassImpl.kt:53)
at kotlin.reflect.jvm.internal.KClassImpl$Data$descriptor$2.invoke(KClassImpl.kt:44)
at kotlin.reflect.jvm.internal.ReflectProperties$LazySoftVal.invoke(ReflectProperties.java:92)
at kotlin.reflect.jvm.internal.ReflectProperties$Val.getValue(ReflectProperties.java:31)
at kotlin.reflect.jvm.internal.KClassImpl$Data.getDescriptor(KClassImpl.kt)
at kotlin.reflect.jvm.internal.KClassImpl$Data$objectInstance$2.invoke(KClassImpl.kt:106)
at kotlin.reflect.jvm.internal.ReflectProperties$LazyVal.invoke(ReflectProperties.java:62)
at kotlin.reflect.jvm.internal.ReflectProperties$Val.getValue(ReflectProperties.java:31)
at kotlin.reflect.jvm.internal.KClassImpl$Data.getObjectInstance(KClassImpl.kt)
at kotlin.reflect.jvm.internal.KClassImpl.getObjectInstance(KClassImpl.kt:239)
at io.ktor.server.engine.ApplicationEngineEnvironmentReloading.createModuleContainer(ApplicationEngineEnvironmentReloading.kt:328)
at io.ktor.server.engine.ApplicationEngineEnvironmentReloading.executeModuleFunction(ApplicationEngineEnvironmentReloading.kt:317)
at io.ktor.server.engine.ApplicationEngineEnvironmentReloading.instantiateAndConfigureApplication(ApplicationEngineEnvironmentReloading.kt:275)
at io.ktor.server.engine.ApplicationEngineEnvironmentReloading.createApplication(ApplicationEngineEnvironmentReloading.kt:127)
at io.ktor.server.engine.ApplicationEngineEnvironmentReloading.start(ApplicationEngineEnvironmentReloading.kt:247)
at io.ktor.server.tomcat.TomcatApplicationEngine.start(TomcatApplicationEngine.kt:108)
at io.ktor.server.tomcat.EngineMain.main(EngineMain.kt:16)
```

This error just happen when trying to execute the FatJar in command line, everything works fine when running in Intellij. I have seen similar issues at:
* https://github.com/ktorio/ktor/issues/386
* https://stackoverflow.com/questions/54282790/ktor-running-fat-jar-throws-java-lang-unsupportedoperationexceptionpackages-an

By changing the `mainClassName` property to correspond to my own Application class, everything seems to work fine! I am not sure if the documentation needs to be updated in [this page](https://ktor.io/servers/deploy/packing/fatjar.html) or if I did something wrong.

Hope I receive an answer from you!

Open the FatJar documentation at ktor.io/servers/deploy/packing/fatjar.html and compare its mainClassName instructions with the reported java -jar failure. Verify the documented entry-point configuration against the working Application class setup, then update the page so the command-line FatJar steps and expected main class are unambiguous.

Written by the indexing model from the issue text.