doc: change “substantial” to “significant”

Change-Id: Ie7f2890eab9ad82da93babc92380b13c611af3d0
Reviewed-on: https://go-review.googlesource.com/83375Reviewed-by: Andrew Bonventre <andybons@golang.org>

doc/go1.10.html

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