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/83356Reviewed-by: Andrew Gerrand <adg@golang.org>

doc/go1.10.html

@@ -36,7 +36,7 @@ OVERVIEW HERE
 <h2 id="language">Changes to the language</h2>
 
 <p>
-There are no substantive changes to the language.
+There are no substantial changes to the language.
 </p>
 
 <p><!-- CL 60230 -->