spec: allow disallow of \uFFFD in import path

Turns out U+FFFD is a graphic character too.

http://play.golang.org/p/YblCfKAfko

R=gri
CC=golang-dev
https://golang.org/cl/5699052

doc/go_spec.html

@@ -5101,8 +5101,9 @@ Implementation restriction: A compiler may restrict ImportPaths to
 non-empty strings using only characters belonging to
 <a href="http://www.unicode.org/versions/Unicode6.0.0/">Unicode's</a>
 L, M, N, P, and S general categories (the Graphic characters without
-spaces) and may also exclude the ASCII characters
-<code>!"#$%&amp;'()*,:;&lt;=&gt;?[\]^`{|}</code>.
+spaces) and may also exclude the characters
+<code>!"#$%&amp;'()*,:;&lt;=&gt;?[\]^`{|}</code>
+and the Unicode replacement character U+FFFD.
 </p>
 
 <p>