math/cmplx: fix typo in code comment
Everywhere else is using "cancellation" as of 2019
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: I933ea68d7251986ce582b92c33b7cb13cee1d207
GitHub-Last-Rev:
fc3d5ada2bd0087ea9cfb3f105689876e7a2ee4f
GitHub-Pull-Request: golang/go#38661
Reviewed-on: https://go-review.googlesource.com/c/go/+/230199
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>