[test/test_all_urls] Update pbs extractor name

It's in lowercase now (since e15e2ef7a0).

test/test_all_urls.py

@@ -121,8 +121,8 @@ class TestAllURLsMatching(unittest.TestCase):
 
     def test_pbs(self):
         # https://github.com/rg3/youtube-dl/issues/2350
-        self.assertMatch('http://video.pbs.org/viralplayer/2365173446/', ['PBS'])
-        self.assertMatch('http://video.pbs.org/widget/partnerplayer/980042464/', ['PBS'])
+        self.assertMatch('http://video.pbs.org/viralplayer/2365173446/', ['pbs'])
+        self.assertMatch('http://video.pbs.org/widget/partnerplayer/980042464/', ['pbs'])
 
     def test_yahoo_https(self):
         # https://github.com/rg3/youtube-dl/issues/2701