spec: clarify wording on passing slice arguments to variadic functions

Per discussion on #30769.

Fixes #30769.

Change-Id: I620dbac936de1a0b5deec03926dd11d690a918e5
Reviewed-on: https://go-review.googlesource.com/c/go/+/167380Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Rob Pike <r@golang.org>

doc/go_spec.html

 <!--{
 	"Title": "The Go Programming Language Specification",
-	"Subtitle": "Version of March 12, 2019",
+	"Subtitle": "Version of March 13, 2019",
 	"Path": "/ref/spec"
 }-->
 
@@ -3484,7 +3484,7 @@ within <code>Greeting</code>, <code>who</code> will have the value
 </p>
 
 <p>
-If the final argument is assignable to a slice type <code>[]T</code>, it may be
+If the final argument is assignable to a slice type <code>[]T</code>, it is
 passed unchanged as the value for a <code>...T</code> parameter if the argument
 is followed by <code>...</code>. In this case no new slice is created.
 </p>