Reword some comments in `re2.h`.

Specifically, say "submatch" instead of "substring" in the context
of supplying extra pointer arguments to `RE2::FullMatch()` et al.

Fixes #394.

Change-Id: Ic41510e0707b0a769e1254087b5e690379f3f0c4
Reviewed-on: https://code-review.googlesource.com/c/re2/+/60670
Reviewed-by: Perry Lorier <perryl@google.com>
Reviewed-by: Paul Wankadia <junyer@google.com>
1 file changed