colorize: fix check for pygmentize (#7250)

plugins/colorize/colorize.plugin.zsh

@@ -7,9 +7,9 @@
 alias ccat='colorize_via_pygmentize'
 
 colorize_via_pygmentize() {
-    if [ ! -x "$(which pygmentize)" ]; then
-        echo "package \'Pygments\' is not installed!"
-        return -1
+    if ! (( $+commands[pygmentize] )); then
+        echo "package 'Pygments' is not installed!"
+        return 1
     fi
 
     if [ $# -eq 0 ]; then