Call `help` instead of `--help`

`--help` is only available on newer versions of nvm, `help` is backwards compatible

zsh-nvm.plugin.zsh

@@ -62,7 +62,7 @@ nvm_update() {
 _zsh_nvm_upgrade() {
 
   # Use default upgrade if it's built in
-  if _zsh_nvm_nvm --help | grep -q 'nvm upgrade'; then
+  if [[ "$(_zsh_nvm_nvm help | grep 'nvm upgrade')" ]]; then
     _zsh_nvm_nvm upgrade
     return
   fi