slog: fix Logger.Error doc

The error Attr now comes first, not last.

Change-Id: I595cb69c15ee07fa345cc2efae7e108bce606943
Reviewed-on: https://go-review.googlesource.com/c/exp/+/467439
Run-TryBot: Jonathan Amsterdam <jba@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>

slog/logger.go

@@ -199,8 +199,8 @@ func (l *Logger) Warn(msg string, args ...any) {
 }
 
 // Error logs at LevelError.
-// If err is non-nil, Error appends Any(ErrorKey, err)
-// to the list of attributes.
+// If err is non-nil, Error adds Any(ErrorKey, err)
+// before the list of attributes.
 func (l *Logger) Error(msg string, err error, args ...any) {
 	l.logDepthErr(err, 1, LevelError, msg, args...)
 }