spec: improve wording by choosing an official term "keyword"
spec: improve wording by choosing an official term "keyword"

Replace "reserved word" by "keyword" as the latter is the official term.

Change-Id: I9f269759b872026034a9f47e4a761cff2d348ca0
Reviewed-on: https://go-review.googlesource.com/c/go/+/323729
Reviewed-by: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>
Trust: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Trust: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>

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