time: fix example in monotonic time comment

t.Sub(u) would be -20 milliseconds.
The right computation is u.Sub(t), but rewrite to be even clearer.

Thanks to Karsten Weiss for catching this.

Change-Id: I6e274d69b0301840d57c5c65bf4114da0d33bf10
Reviewed-on: https://go-review.googlesource.com/46971
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/time/time.go

@@ -23,10 +23,10 @@
 // approximately 20 milliseconds, even if the wall clock is changed during
 // the operation being timed:
 //
-//	t := time.Now()
+//	start := time.Now()
 //	... operation that takes 20 milliseconds ...
-//	u := time.Now()
-//	elapsed := t.Sub(u)
+//	t := time.Now()
+//	elapsed := t.Sub(start)
 //
 // Other idioms, such as time.Since(start), time.Until(deadline), and
 // time.Now().Before(deadline), are similarly robust against wall clock