methods: Remove requirement about name length

We already have registered methods that violate this requirement;
and methods should be free to make their own choices about this.
For #403

index.html

@@ -3007,10 +3007,6 @@ <h2>Method Schemes</h2>
 publication.
       </p>
 
-      <p>
-The method name SHOULD be five characters or less.
-      </p>
-
       <div class="note" title="Unique DID method names">
         <p>
 Because there is no central authority for allocating or approving <a>DID