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

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

re2/tostring.cc

diff --git a/re2/tostring.cc b/re2/tostring.cc
index 4545a92..9c1c038 100644
--- a/re2/tostring.cc
+++ b/re2/tostring.cc
@@ -291,7 +291,7 @@
       // There's no syntax accepted by the parser to generate
       // this node (it is generated by RE2::Set) so make something
       // up that is readable but won't compile.
-      t_->append("(?HaveMatch:%d)", re->match_id());
+      t_->append(StringPrintf("(?HaveMatch:%d)", re->match_id()));
       break;
   }