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.

libexec/rbenv-help

@@ -64,8 +64,8 @@ collect_documentation() {
     }
 
     function trim(str) {
-      gsub(/^\n*/, "", str)
+      sub(/^\n*/, "", str)
-      gsub(/\n*$/, "", str)
+      sub(/\n*$/, "", str)
       return str
     }