minor docs readme change

docs/README.md

@@ -2,7 +2,7 @@
 
 These are Sphinx docs, automatically built at http://docs.wagtail.io when the master branch is committed to Github. To build them locally, install Wagtail's development requirements (in the root Wagtail directory):
 
-    pip install requirements-dev.txt
+    pip install -r requirements-dev.txt
 
 To build the documentation for browsing, from this directory run: