strconv: add missing period to godoc comment

Change-Id: I90ba0a6e0c6ccdce16938eed09424308a84fc6fb
GitHub-Last-Rev: 66b6db1a674e6817209a69a7ccd1846d3b0e1900
GitHub-Pull-Request: golang/go#25801
Reviewed-on: https://go-review.googlesource.com/117575Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/strconv/atob.go

@@ -17,7 +17,7 @@ func ParseBool(str string) (bool, error) {
 	return false, syntaxError("ParseBool", str)
 }
 
-// FormatBool returns "true" or "false" according to the value of b
+// FormatBool returns "true" or "false" according to the value of b.
 func FormatBool(b bool) string {
 	if b {
 		return "true"