Documentation for word count example is unclear about inputFile vs pom.xml

Currently the documentation for the word count example mentions the input file and give the following example:

```Java
$ mvn compile exec:java -Dexec.mainClass=org.apache.beam.examples.WordCount \
-Dexec.args="\--inputFile=pom.xml \--output=counts" -Pdirect-runner
``
. As far as I believe the pom.xml is not supposed to be the input file in this case. I will create a PR if someone could confirm that my assumption is correct :)

/Niklas

Imported from Jira [BEAM-7976](https://issues.apache.org/jira/browse/BEAM-7976). Original Jira may contain additional context.
Reported by: nikenano.