Call openapiYmlRegister() instead of openapiRegister()

Hi,
(Really great tool you've written, thanks)

The readme.md says to call openapiRegister() from main.go, but I have to call openapiYmlRegister() instead.

I don't know if this is a bug or what, but since I'm not fluent in go, it took me some time to track down the issue.

I was getting the following error message:
```
# command-line-arguments
./main.go:16:9: undefined: openapiRegister
```