One more fix to dev docs (astropy#1112)

* Update contributing.rst

Needed single quotes in order for pip install command to work.

* Update contributing.rst

pin python version to 3.11

docs/contributing.rst

@@ -67,10 +67,10 @@ Ready to contribute? Here's how to set up :ref:`specutils <specutils>` for local
 3. Install your local copy, preferably into some sort of virtual environment using your
    preferred environment manager. For example, using conda::
 
-    $ conda create --name specutils-dev python pip
+    $ conda create --name specutils-dev python=3.11 pip
     $ conda activate specutils-dev
     $ cd specutils/
-    $ pip install -e .[test]
+    $ pip install -e .'[test]'
 
 4. Create a branch for local development::