src/sage/doctest/forker.py: change the slow doctest warning punctuation

Switch "Warning, slow doctest:" to "Warning: slow doctest:" so that
the leading "Warning: " is consistent with other warnings. This
ultimately prevents these slow doctest warnings from being converted
into error annotations on the Github CI.

src/sage/doctest/forker.py

@@ -1537,12 +1537,12 @@ def report_overtime(self, out, test, example, got, *, check_timer=None):
             sage: DTR.report_overtime(sys.stdout.write, doctests[0], ex, 'BAD ANSWER\n', check_timer=check)
             **********************************************************************
             File ".../sage/doctest/forker.py", line 12, in sage.doctest.forker
-            Warning, slow doctest:
+            Warning: slow doctest:
                 doctest_var = 42; doctest_var^2
             Test ran for 1.23s cpu, 2.50s wall
             Check ran for 2.34s cpu, 3.12s wall
         """
-        out(self._failure_header(test, example, 'Warning, slow doctest:') +
+        out(self._failure_header(test, example, 'Warning: slow doctest:') +
             ('Test ran for %.2fs cpu, %.2fs wall\nCheck ran for %.2fs cpu, %.2fs wall\n'
              % (example.cputime,
                 example.walltime,