Update error message in Makefile

The file this message refers to was renamed in
commit 53b3b93.

Author: mjdominus

doc/Makefile

@@ -10,7 +10,7 @@ BUILDDIR      = _build
 
 # Put it first so that "make" without argument is like "make help".
 help:
-	@if [ ! -d venv ]; then echo "WARNING: There is no venv directory, did you forget to 'virtualenv venv'. Check building-the-docs file."; fi
+	@if [ ! -d venv ]; then echo "WARNING: There is no venv directory, did you forget to 'virtualenv venv'. Check README.md."; fi
 	@if [ ! "z$$(which $(SPHINXBUILD))" = "z$$(pwd)/venv/bin/sphinx-build" ]; then echo "WARNING: Did you forgot to 'source venv/bin/activate'"; fi
 	@$(SPHINXBUILD) -M help "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(O)