Fix bug where mm is mistakenly replaced with hmm in e.g. 20mm (#659)

Co-authored-by: Jong Wook Kim <jongwook@nyu.edu>

tests/test_normalizer.py

@@ -84,6 +84,7 @@ def test_text_normalizer():
     assert std("he's like") == "he is like"
     assert std("she's been like") == "she has been like"
     assert std("10km") == "10 km"
+    assert std("10mm") == "10 mm"
     assert std("RC232") == "rc 232"
 
     assert (