fix single word case (#1679)

彭震东

2024-04-29 1f49a4f2b2744c30fcbfff6fbb85d6133c6203f2

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p m    p.m.
pm    p.m.
p.m.
p.m    p.m.
am    a.m.
a.m.
a.m    a.m.
a m    a.m.