.osx: Add missing value

In order to properly overwrite the `key` of a `domain`, `defaults` needs to be provided with a valid `value` for the `key`:

    `defaults write <domain> <key> <value>`

Closes mathiasbynens#350.

.osx

@@ -354,7 +354,7 @@ defaults write com.apple.dock show-process-indicators -bool true
 # Wipe all (default) app icons from the Dock
 # This is only really useful when setting up a new Mac, or if you don’t use
 # the Dock to launch apps.
-#defaults write com.apple.dock persistent-apps -array
+#defaults write com.apple.dock persistent-apps -array ""
 
 # Don’t animate opening applications from the Dock
 defaults write com.apple.dock launchanim -bool false