Fix 'make help' (#12509)

These targets are actually using plural names, fix help output for them.

Ref: https://github.com/go-gitea/gitea/pull/12352

Makefile

@@ -161,9 +161,9 @@ help:
 	@echo " - lint                             lint everything"
 	@echo " - lint-frontend                    lint frontend files"
 	@echo " - lint-backend                     lint backend files"
-	@echo " - check                            run various consistency checks"
+	@echo " - checks                           run various consistency checks"
-	@echo " - check-frontend                   check frontend files"
+	@echo " - checks-frontend                  check frontend files"
-	@echo " - check-backend                    check backend files"
+	@echo " - checks-backend                   check backend files"
 	@echo " - webpack                          build webpack files"
 	@echo " - svg                              build svg files"
 	@echo " - fomantic                         build fomantic files"