Use more reliable installation command in docs

Some shell environments may interpret brackets as glob patterns.
Wrapping the argument in quotation marks should help ensure correct
behavior in those environments. Refs #2786

docs/install.rst

@@ -11,7 +11,7 @@ You can install Pelican via several different methods. The simplest is via
 
 Or, if you plan on using Markdown::
 
-    pip install pelican[Markdown]
+    pip install "pelican[markdown]"
 
 (Keep in mind that operating systems will often require you to prefix the above
 command with ``sudo`` in order to install Pelican system-wide.)
@@ -46,7 +46,7 @@ Optional packages
 If you plan on using `Markdown <https://pypi.org/project/Markdown/>`_ as a
 markup format, you can install Pelican with Markdown support::
 
-   pip install pelican[Markdown]
+    pip install "pelican[markdown]"
 
 Or you might need to install it a posteriori::
 

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]
+    pip install "pelican[markdown]"
 
 Create a project
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