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/quickstart.rst

@@ -11,7 +11,7 @@ Install Pelican (and optionally Markdown if you intend to use it) on Python
 2.7.x or Python 3.5+ by running the following command in your preferred
 terminal, prefixing with ``sudo`` if permissions warrant::
 
-    pip install "pelican[markdown]"
+    python -m pip install "pelican[markdown]"
 
 Create a project
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