Merge pull request #907 from apenney/typo

Fix a small typo.

docs/index.md

@@ -21,7 +21,7 @@ reproduced anywhere:
 
     FROM python:2.7
     WORKDIR /code
-    ADD rements.txt /code/
+    ADD requirements.txt /code/
     RUN pip install -r requirements.txt
     ADD . /code