Fix incorrect formatting of rbenv-help output under MAWK

In systems that use the MAWK interpreter (the default AWK installed with
Ubuntu), the output of `rbenv help <command>` would have no line breaks.
The issue is fixed by changing `gsub` to `sub` in the snippet of awk
commands that are used to extract documentation comments.

I suspect the bug is something to do with the way the '^' and '$'
characters are interpreted by different AWK interpreters (per-line vs
per-string anchors).

If I understand correctly, the purpose of trim() is to remove all line
breaks from the start and end of each sections of a command's
documentation, in which case `sub` should serve the same purpose.
Merge pull request #310 from leocassarani/rbenv-help-mawk

Fix incorrect formatting of rbenv-help output under MAWK