doc: clarify a sentence about *_js.go
doc: clarify a sentence about *_js.go

Change "have to" to "need to" for clarity and to avoid a
peculiar English idiom.

Change-Id: Iec2b1f841d0353dd7925f8f934fe82d4ed059d7d
Reviewed-on: https://go-review.googlesource.com/123495
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

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