[MRG] fix command for make test-coverage (scikit-learn#10188)

Author: amueller

Makefile

@@ -34,7 +34,7 @@ endif
 
 test-coverage:
 	rm -rf coverage .coverage
-	$(PYTEST) sklearn --show-locals -v --with-cov sklearn
+	$(PYTEST) sklearn --showlocals -v --cov=sklearn --cov-report=html:coverage
 
 test: test-code test-sphinxext test-doc