net/http: rename server receiver for consistency

"receiver name `srv` should be consistent with the previous receiver name
`s` for Server" according to go-lint.

Change-Id: I3071ae30b7c1375999e5a599a0eee7b88494f17e
GitHub-Last-Rev: 093c767
GitHub-Pull-Request: #65787
Reviewed-on: https://go-review.googlesource.com/c/go/+/565175
Auto-Submit: Damien Neil <[email protected]>
Reviewed-by: Carlos Amedee <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Damien Neil <[email protected]>

Author: WineChord