Adjust a thread annotation.
Adjust a thread annotation.

Change-Id: Ib7867190116fbc65d404804c4f14392f599d8bd3
Reviewed-on: https://code-review.googlesource.com/c/re2/+/44510
Reviewed-by: Paul Wankadia <62d4e38bb1380f055ec4b899baa75e52af09c5f7@google.com>

google/re2,google/re2,google/re2,google/re2,google/re2