pull retropad out of handwired and update readme.md (#11545)

Co-authored-by: Swiftrax <swiftrax@gmail.com>

keyboards/handwired/swiftrax/retropad/readme.md → keyboards/swiftrax/retropad/readme.md

@@ -8,6 +8,6 @@
 
 Make example for this keyboard (after setting up your build environment):
 
-    make handwired/swiftrax/retropad:default
+    make swiftrax/retropad:default
 
 See the [build environment setup](https://docs.qmk.fm/#/getting_started_build_tools) and the [make instructions](https://docs.qmk.fm/#/getting_started_make_guide) for more information. Brand new to QMK? Start with our [Complete Newbs Guide](https://docs.qmk.fm/#/newbs).