net/http/httputil: fix typo in unit test name
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 <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Run-TryBot: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

cloudflare/go,CAFxX/go,akutz/go,golang/go,CAFxX/go,cloudflare/go,nawawi/go,nawawi/go,cloudflare/go,nawawi/go,golang/go,akutz/go,CAFxX/go,cloudflare/go,CAFxX/go,akutz/go,akutz/go,golang/go,nawawi/go,CAFxX/go,CAFxX/go,golang/go,nawawi/go,akutz/go,golang/go,CAFxX/go,akutz/go,golang/go,cloudflare/go,cloudflare/go,CAFxX/go,golang/go,cloudflare/go,akutz/go,cloudflare/go,akutz/go,nawawi/go,nawawi/go,golang/go,nawawi/go