[go/en][go/es] Fix several problems with Go tutorials

I have updated go/en and go/es with the following changes:
* Fix a bug with the declaration of learnVariadicParams which should require an interface{}, not a string.  Errors were thrown until this was corrected.
* Clarify that variables declared in blocks are local to that block (solves #504).
* Generally clean up comments, mostly formatting.