Fix pyenv -> rbenv typo

completions/rbenv.fish

@@ -18,6 +18,6 @@ end
 
 complete -f -c rbenv -n '__fish_rbenv_needs_command' -a '(rbenv commands)'
 for cmd in (rbenv commands)
-  complete -f -c pyenv -n "__fish_pyenv_using_command $cmd" -a \
+  complete -f -c rbenv -n "__fish_rbenv_using_command $cmd" -a \
     "(rbenv completions (commandline -opc)[2..-1])"
 end