Minor update to docs.

docs/installation.rst

@@ -34,7 +34,7 @@ with installation of the pre-built binary, you can compile from source. This
 is the best option in general because it ensures the package is optimized
 for your particular platform but is slightly trickier. To do this, run:::
 
-  pip install xGPR --no-binary --verbose
+  pip install --no-binary :all: xGPR --verbose
 
 We recommend running with the ``--verbose`` flag since this will ensure that
 any error messages generated by CMake as it tries to compile the code are