Update serve_command.dart

I like to just copy and pasted when these suggestions pop up but right now I have to remember the "dart" precursor on my own. I figured I'd just add it in here.

Author: kimlarson-wk

lib/src/serve_command.dart

@@ -82,14 +82,14 @@ class ServeCommand extends Command<int> {
       return red.wrap(
         'This command requires that `webdev` be activated globally.\n'
         'Please run the following:\n'
-        '\tpub global activate webdev "$webdevCompatibility"',
+        '\tdart pub global activate webdev "$webdevCompatibility"',
       );
     } else if (!_hasCompatibleWebdev) {
       return red.wrap(
         'This command is only compatible with `webdev $webdevCompatibility`, '
         'but you currently have webdev ${getGlobalWebdevVersion()} active.\n'
         'Please run the following to activate a compatible version:\n'
-        '\tpub global activate webdev "$webdevCompatibility"',
+        '\tdart pub global activate webdev "$webdevCompatibility"',
       );
     }
     return null;