Fix to building instructions

`.  /env/activate` does not work when building the documentation. Changed with `. /env/bin/activate'

Signed-off-by: restelli <arestell@umd.edu>
diff --git a/README.rst b/README.rst
index f943fa4..f225b43 100644
--- a/README.rst
+++ b/README.rst
@@ -89,7 +89,7 @@
 
    # Create a Python virtual environment and install requirements into it.
    virtualenv env --python=python3
-   . env/activate
+   . env/bin/activate
    pip install -r requirements.txt
 
    # Build the documentation