Fix a bug in `Regexp::ToString()`.
Fix a bug in `Regexp::ToString()`.

Change-Id: Ib02554e460dbf45502bcab8798a0c1659c5ff83d
Reviewed-on: https://code-review.googlesource.com/c/re2/+/58630
Reviewed-by: Paul Wankadia <62d4e38bb1380f055ec4b899baa75e52af09c5f7@google.com>

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