Makefile: fix target check-black

Fix a typo to mark target check-black as PHONY.

Signed-off-by: Sachin Prabhu <[email protected]>

Author: spuiuk

Makefile

@@ -26,7 +26,7 @@ check-mypy:
 check-flake8:
 	$(call runtox, "flake8")
 
-PHONY: check-black
+.PHONY: check-black
 check-black:
 	$(call runtox, "black")