log: fix comment

I just want to experience the whole Gerrit Flow, so I make this simple commit
as my first commit to golang src repo.

Change-Id: Ie744573beac7a8b9361f898fac269c9d88010493
Reviewed-on: https://go-review.googlesource.com/48630Reviewed-by: Ma Peiqi <mapeiqi2017@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/log/log.go

@@ -157,7 +157,7 @@ func (l *Logger) Output(calldepth int, s string) error {
 	l.mu.Lock()
 	defer l.mu.Unlock()
 	if l.flag&(Lshortfile|Llongfile) != 0 {
-		// release lock while getting caller info - it's expensive.
+		// Release lock while getting caller info - it's expensive.
 		l.mu.Unlock()
 		var ok bool
 		_, file, line, ok = runtime.Caller(calldepth)