DOC: Correct nightly wheels pip install command

Add the --pre flag to allow for a dev release to take precedence over
a stable release and the --extra-index-url of https://pypi.org/simple
to allow for matplotlib's dependencies to still get installed normally.

Co-authored-by: Ian Hunt-Isaak <[email protected]>

Author: matthewfeickert

doc/users/installing/index.rst

@@ -92,7 +92,12 @@ as the package index to query:
 
 .. code-block:: sh
 
-  python -m pip install --upgrade --index-url https://pypi.anaconda.org/scipy-wheels-nightly/simple matplotlib
+  python -m pip install \
+    --upgrade \
+    --pre \
+    --index-url https://pypi.anaconda.org/scipy-wheels-nightly/simple \
+    --extra-index-url https://pypi.org/simple \
+    matplotlib
 
 ======================
 Installing from source