Fix bug in Ngram splitting logic

Rather than returning the TemporarySpan, along with its splits, Snorkel
was returning the TemporarySpan twice, and only the 2nd split. Hiromu
Hota fixed this bug in Fonduer in [1]. This commit fixes it for Snorkel.

[1] HazyResearch/fonduer#108

Co-authored-by: Hiromu Hota <hiromu.hota@hal.hitachi.com>

snorkel/candidates.py

@@ -172,7 +172,7 @@ def apply(self, context):
                         ts1 = TemporarySpan(char_start=start, char_end=start + m.start(1) - 1, sentence=context)
                         if ts1 not in seen:
                             seen.add(ts1)
-                            yield ts
+                            yield ts1
                         ts2 = TemporarySpan(char_start=start + m.end(1), char_end=end, sentence=context)
                         if ts2 not in seen:
                             seen.add(ts2)