doc/go1.15: document no language changes

There are no language changes in Go 1.15, so document that.

For #37419.

Change-Id: I1e96e58b701f1758d64c79881dfa0b1109836b83
Reviewed-on: https://go-review.googlesource.com/c/go/+/235580
Reviewed-by: Robert Griesemer <[email protected]>

Author: dmitshur

doc/go1.15.html

@@ -26,7 +26,7 @@ <h2 id="introduction">DRAFT RELEASE NOTES — Introduction to Go 1.15</h2>
 <h2 id="language">Changes to the language</h2>
 
 <p>
-TODO
+  There are no changes to the language.
 </p>
 
 <h2 id="ports">Ports</h2>