net/http/httputil: fix typo in unit test name
Everywhere else is using "cancellation"
The reasoning is mentioned in 170060
> Though there is variation in the spelling of canceled,
> cancellation is always spelled with a double l.
>
> Reference: https://www.grammarly.com/blog/canceled-vs-cancelled/
Change-Id: Ifc97c6785afb401814af77c377c2e2745ce53c5a
GitHub-Last-Rev:
05edd7477da46ca5c02703e139dbfc0cd05408ac
GitHub-Pull-Request: golang/go#38662
Reviewed-on: https://go-review.googlesource.com/c/go/+/230200
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>