doc/go1.13: remove 1.12 reference from 1.13 release notes

While it's true that CL 168479 was also backported to 1.12, this is
irrelevant for the Go1.13 release notes.

Change-Id: I044be69f8f2bc005f58a0d63db05129add15f674
Reviewed-on: https://go-review.googlesource.com/c/go/+/186138Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/go1.13.html

@@ -790,8 +790,9 @@ godoc
 <dl id="syscall"><dt><a href="/pkg/syscall/">syscall</a></dt>
   <dd>
     <p><!-- CL 168479 -->
-      Uses of <code>_getdirentries64</code> have been removed from Darwin builds, to allow binaries
-      built with 1.12 to be uploaded to the macOS App Store.
+      Uses of <code>_getdirentries64</code> have been removed from
+      Darwin builds, to allow Go binaries to be uploaded to the macOS
+      App Store.
     </p>
 
     <p><!-- CL 174197 -->