src/sage/doctest/control.py: update _init_warn_long() docstring

Now that the --warn-long defaults are 5.0 and 30.0 seconds for regular
and long tests respectively, we update the _init_warn_long() docstring
to reflect that.

src/sage/doctest/control.py

@@ -582,8 +582,8 @@ def _init_warn_long(self):
         finish in less than about 5 seconds. Longer tests typically
         don't add coverage, they just make testing slow.
 
-        The default used here is 10 seconds, unless `--long` was used,
-        in which case it is 60 seconds.
+        The default used here is 5 seconds, unless `--long` was used,
+        in which case it is 30 seconds.
 
         TESTS: