regexp: add the missing is

Change-Id: I23264972329aa3414067cd0e0986b69bb39bbeb5
GitHub-Last-Rev: d1d668a
GitHub-Pull-Request: #50650
Reviewed-on: https://go-review.googlesource.com/c/go/+/378935
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Daniel Martí <mvdan@mvdan.cc>

Author: luochuanhang

src/regexp/regexp.go

@@ -42,7 +42,7 @@
 // successive submatches of the expression. Submatches are matches of
 // parenthesized subexpressions (also known as capturing groups) within the
 // regular expression, numbered from left to right in order of opening
-// parenthesis. Submatch 0 is the match of the entire expression, submatch 1
+// parenthesis. Submatch 0 is the match of the entire expression, submatch 1 is
 // the match of the first parenthesized subexpression, and so on.
 //
 // If 'Index' is present, matches and submatches are identified by byte index