doc/go_spec: tweak wording to avoid 'explicit assignment' misreading

This text changed in CL 139099 to add "explicit" in front of "conversion".
But now "explicit conversion or assignment" reads like it might mean
"explicit [conversion or assignment]" when what is meant is
"[explicit conversion] or assignment". To make clear that explicit does
not apply to assignment, use "assignment or explicit conversion".

Change-Id: I8ff7a5b3ecd9f562793502fa6808242f22264f28
Reviewed-on: https://go-review.googlesource.com/c/149340Reviewed-by: Robert Griesemer <gri@golang.org>

doc/go_spec.html

 <!--{
 	"Title": "The Go Programming Language Specification",
-	"Subtitle": "Version of November 12, 2018",
+	"Subtitle": "Version of November 16, 2018",
 	"Path": "/ref/spec"
 }-->
 
@@ -1348,8 +1348,9 @@ ChannelType = ( "chan" | "chan" "&lt;-" | "&lt;-" "chan" ) ElementType .
 The optional <code>&lt;-</code> operator specifies the channel <i>direction</i>,
 <i>send</i> or <i>receive</i>. If no direction is given, the channel is
 <i>bidirectional</i>.
-A channel may be constrained only to send or only to receive by explicit
-<a href="#Conversions">conversion</a> or <a href="#Assignments">assignment</a>.
+A channel may be constrained only to send or only to receive by
+<a href="#Assignments">assignment</a> or
+explicit <a href="#Conversions">conversion</a>.
 </p>
 
 <pre>