Update Levinson Readme to reflect changes to directory structure (#7883)

Updates the example make commands in the Levinson readme to
keebio/levinson/...  instead of levinson/...

Also updated the example to rev3 (the current version of the board)

keyboards/keebio/levinson/readme.md

@@ -9,11 +9,11 @@ Hardware Availability: [Keebio](https://keeb.io)
 
 Make example for this keyboard (after setting up your build environment):
 
-    make levinson/rev2:default
+    make keebio/levinson/rev3:default
 
 Example of flashing this keyboard with Pro micro:
 
-    make levinson/rev2:default:avrdude
+    make keebio/levinson/rev3:default:avrdude
 
 Example of flashing this keyboard with Elite-C:
 
@@ -21,7 +21,7 @@ You'll need to add `BOOTLOADER = atmel-dfu` to your `rules.mk` to ensure that re
 
 Hold the reset button on the Elite-C board, or the reset button on the keyboard if already assembled, then release the button and run:
 
-    make levinson/rev2:default:dfu
+    make keebio/levinson/rev3:default:dfu
 
 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).