Correct docstring for Panel.enable_instrumentation()

Since 5299cf8 the method
DebugToolbarMiddleware.process_request() does not exist.

Also drop the Sphinx :class: directive as DebugToolbarMiddleware doesn't
have a definition in docs.

Author: jdufresne

debug_toolbar/panels/__init__.py

@@ -131,8 +131,7 @@ def enable_instrumentation(self):
         time.
 
         Unless the toolbar or this panel is disabled, this method will be
-        called early in :class:`DebugToolbarMiddleware.process_request`. It
-        should be idempotent.
+        called early in ``DebugToolbarMiddleware``. It should be idempotent.
         """
 
     def disable_instrumentation(self):