Update PEP 101: version_switch.js -> switchers.js

The old name "version_switch.js" is completely dead.
Only the new name is in use in all active branches.
So there's no need to keep the old name floating around
in a lively, actively maintained, devastatingly current
document like PEP 101.

pep-0101.txt

@@ -464,7 +464,7 @@ to perform some manual editing steps.
     the `docsbuild-scripts <https://github.com/python/docsbuild-scripts>`_
     to point to the right stable/development branches, and to install it
     and make the initial checkout.
-    The Doc's ``version_switcher.js`` script also needs to be
+    The Doc's ``switchers.js`` script also needs to be
     updated.  In general, please don't touch things in the toplevel
     ``/srv/docs.python.org/`` directory unless you know what you're doing.
 
@@ -830,7 +830,7 @@ with RevSys.)
   These include:
 
   - Update the issue tracker for the new branch.
- 
+
       * Add the new version to the versions list (contact the tracker
         admins?).
 
@@ -911,7 +911,7 @@ else does them.  Some of those tasks include:
   retired branch in the Doc version switcher and side bar templates
   (file names may vary between branches) [DE]::
 
-        Doc/tools/static/version_switch.js
+        Doc/tools/static/switchers.js
         Doc/tools/templates/indexsidebar.html
 
 - Freeze the state of the release branch by creating a tag of its current HEAD