MUST NOT => SHOULD NOT on URI schemes.

Fixes #310.

Author: gkellogg

index.html

@@ -11776,7 +11776,7 @@ <h2>Terms</h2>
 
     <p class="changed">When used as the <a>prefix</a> in a <a>Compact IRI</a>, to avoid
       the potential ambiguity of a <a>prefix</a> being confused with an IRI
-      scheme, <a>terms</a> MUST NOT come from the list of URI schemes as defined in
+      scheme, <a>terms</a> SHOULD NOT come from the list of URI schemes as defined in
       [[IANA-URI-SCHEMES]]. Similarly, to avoid confusion between a
       <a>Compact IRI</a> and a <a>term</a>, terms SHOULD NOT include a colon (<code>:</code>)
       and SHOULD be restricted to the form of