DOC: Display "dev" instead of "devdocs" in the version switcher

While the url is devdocs historically, and I'm not going to
change this, the displayed names should be "stable" and "dev".
This has the added advantage that dev takes a little less space
in the menubar, and thus wrapping occurs slightly later.

Author: timhoffm

doc/_static/switcher.json

@@ -6,7 +6,7 @@
     },
     {
         "name": "3.7 (dev)",
-        "version": "devdocs",
+        "version": "dev",
         "url": "https://matplotlib.org/devdocs/"
     },
     {