Fix multiple argument completion with zsh

This changes the zsh completion to omit the final, incomplete command
line argument when invoking rbenv completions, making it consistent with
the bash completion.  Since no built-in completion cares about the
argument list, this inconsistency only affected plugins.

completions/rbenv.zsh

@@ -11,7 +11,7 @@ _rbenv() {
   if [ "${#words}" -eq 2 ]; then
     completions="$(rbenv commands)"
   else
-    completions="$(rbenv completions ${words[2,-1]})"
+    completions="$(rbenv completions ${words[2,-2]})"
   fi
 
   reply=("${(ps:\n:)completions}")