docs: wrap `tutor[full]` in quotes for zsh compatibility

The docs recommend commands like:

    pip install tutor[full]
    pip install -e ./tutor[full]

for installing Tutor. These work fine in bash. For zsh,
though, which is now the default on macOS, quotes are
needed, otherwise zsh will interpret the brackets as
special syntax:

    pip install "tutor[full]"
    pip install -e "./tutor[full]"

Caveat: I have not tested this myself since I don't
own a Mac, but I've read several issue reports to this
effect, such as:
https://github.com/pypa/pipenv/issues/2830#issuecomment-419593199

docs/_release_description.md

@@ -1,6 +1,6 @@
 Install this version from pip with::
 
-    pip install tutor[full]==TUTOR_VERSION
+    pip install "tutor[full]==TUTOR_VERSION"
 
 Or download the compiled binaries::
 

docs/download/pip.rst

@@ -1,3 +1,3 @@
 .. parsed-literal::
 
-    pip install tutor[full]
+    pip install "tutor[full]"

docs/install.rst

@@ -89,7 +89,7 @@ Upgrading
 
 To upgrade Open edX or benefit from the latest features and bug fixes, you should simply upgrade Tutor. Start by upgrading the "tutor" package and its dependencies::
 
-    pip install --upgrade tutor[full]
+    pip install --upgrade "tutor[full]"
 
 Then run the ``quickstart`` command again. Depending on your deployment target, run either::