Before: ... appears in an ambiguous URL context.
After:  ... appears in an ambiguous context within a URL.

It's a minor point, but it's confused multiple people.
Try to make clearer that the ambiguity is "where exactly inside the URL?"

Fixes #17319.

Change-Id: Id834868d1275578036c1b00c2bdfcd733d9d2b7b
Reviewed-on: https://go-review.googlesource.com/31465
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>