Prefix Pip commands with python -m in docs

This ensures Pip commands will be executed for the current Python
interpreter and not, say, whichever Python interpreter happens to be
associated with `/usr/local/bin/pip`.

docs/faq.rst

@@ -74,7 +74,7 @@ Markdown format, you will need to explicitly install the Markdown library. You
 can do so by typing the following command, prepending ``sudo`` if permissions
 require it::
 
-    pip install markdown
+    python -m pip install markdown
 
 Can I use arbitrary metadata in my templates?
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