VIM: Casual version - clarification in the "Dangerous commands" additional info.

The [additional info](https://github.com/google/styleguide/blob/35fd3f01678c63593b80c4abbf1691a79fed2dfe/vimscriptguide.xml#L89) mentions that the
help tag for the builtin functions is:
```vim
:help functions()
```
Where in fact it is:
```vim
:help functions
```
or:
```vim
:help function-list
```
Source:
https://github.com/vim/vim/blob/master/runtime/doc/eval.txt#L2299