fix(makefile): point zunit install hint to z-shell/zunit

zunit moved from the retired zdharma org to z-shell. Update the test target's install hint URL.

Author: ss-o

Makefile

@@ -3,4 +3,4 @@ check: test
 test:
 	@command -v zunit >/dev/null && \
 		{ zunit; ((1)); } || \
-		echo "Please install zunit to run the tests (https://github.com/zdharma/zunit)"
+		echo "Please install zunit to run the tests (https://github.com/z-shell/zunit)"