fix zenroom2.vim | bug fix

When I use your vim plugin, the below error appears.

```
Error detected while processing /Users/sota1235/.vim_runtime/sources_non_forked/vim-zenroom2/plugin/zenroom2.vim:
line   65:
E128: Function name must start with a capital or "s:": g:zenroom_goyo_before()
line   67:
E133: :return not inside a function
line   74:
E193: :endfunction not inside a function
line   76:
E128: Function name must start with a capital or "s:": g:zenroom_goyo_after()
line   78:
E133: :return not inside a function
line   88:
E193: :endfunction not inside a function
line   90:
E700: Unknown function: g:zenroom_goyo_before
E700: Unknown function: g:zenroom_goyo_after
Press ENTER or type command to continue
```

So, I fix it.

Please merge pull request.