doc: fix misuse of “substantive” in 1.10 release notes
doc: fix misuse of “substantive” in 1.10 release notes

Use “substantial”, which is believed to be the correct word.
Additionally, this change strips trailing whitespace from the file.

Change-Id: I5b6b718fc09e4b8b911b95e8be0733abd58e165d
Reviewed-on: https://go-review.googlesource.com/83356
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

Cofyc/go,josharian/go,codestation/go,codestation/go,josharian/go,golang/go,akutz/go,CAFxX/go,akutz/go,nawawi/go,CAFxX/go,CAFxX/go,codestation/go,golang/go,akutz/go,CAFxX/go,Cofyc/go,golang/go,cloudflare/go,josharian/go,Cofyc/go,Cofyc/go,golang/go,josharian/go,nawawi/go,cloudflare/go,codestation/go,golang/go,josharian/go,akutz/go,cloudflare/go,CAFxX/go,nawawi/go,Cofyc/go,akutz/go,akutz/go,christopher-henderson/Go,cloudflare/go,nawawi/go,CAFxX/go,golang/go,CAFxX/go,josharian/go,Cofyc/go,codestation/go,codestation/go,josharian/go,nawawi/go,akutz/go,cloudflare/go,golang/go,akutz/go,cloudflare/go,cloudflare/go,Cofyc/go,codestation/go,nawawi/go,nawawi/go,golang/go,Cofyc/go,nawawi/go,codestation/go,cloudflare/go,josharian/go,CAFxX/go