docs: fix a typo in plist_options

While using `"true"` instead of `true` wouldn't break anything, it doesn't
seem to be expected, and using `"false"` instead of `false` _would_ break
things.

Library/Homebrew/formula.rb

@@ -2062,7 +2062,7 @@ class Formula
     # <pre>plist_options :manual => "foo"</pre>
     #
     # Or perhaps you'd like to give the user a choice? Ooh fancy.
-    # <pre>plist_options :startup => "true", :manual => "foo start"</pre>
+    # <pre>plist_options :startup => true, :manual => "foo start"</pre>
     def plist_options(options)
       @plist_startup = options[:startup]
       @plist_manual = options[:manual]