Change target_directory option to -d/--directory as -t was already ta…

…ken by --trace

lib/u3d/commands_generator.rb

@@ -113,7 +113,7 @@ def run
       command :shortcuts do |c|
         c.syntax = 'u3d shortcuts [-u | --unity_version <version>] [-t | --target <folder>]'
         c.option '-u', '--unity_version STRING', String, 'Creates a shortcut for this version of Unity only.'
-        c.option '-t', '--target_directory STRING', String, 'Specifies which directory to create the shortcuts in'
+        c.option '-d', '--directory STRING', String, 'Specifies which directory to create the shortcuts in'
         c.example 'Create a shortcut for all installed versions on the Desktop', 'u3d shortcuts -t ~/Desktop'
         c.example 'Create a shortcut for Unity 5.6.0f3', 'u3d shortcuts -u 5.6.0f3'
         c.summary = 'Create shortcuts for installed versions of Unity'