cmd/racebuild: use multi-line match for README regexp

In https://golang.org/cl/115375 I had neglected to test against a
non-empty README file, and accidentally wrote a regexp that never
matches.

Change-Id: I676cb11abea7e0f5bf337aab640ebc2478295a9b
Reviewed-on: https://go-review.googlesource.com/115377
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

cmd/racebuild/racebuild.go

@@ -392,7 +392,7 @@ func (p *Platform) UpdateReadme() error {
 	// record it in the README.
 	updatedLine := fmt.Sprintf(readmeTmpl, syso, *flagRev, goRev)
 
-	lineRE, err := regexp.Compile("^" + fmt.Sprintf(readmeTmpl, regexp.QuoteMeta(syso), commitRE, commitRE) + "$")
+	lineRE, err := regexp.Compile("(?m)^" + fmt.Sprintf(readmeTmpl, regexp.QuoteMeta(syso), commitRE, commitRE) + "$")
 	if err != nil {
 		return err
 	}