Correct a wrong link for form attribute. (#299)

Author: Paalon

spec.html

@@ -1458,7 +1458,7 @@ <h4>Operator, Fence, Separator or Accent <code>&lt;mo&gt;</code></h4>
               In the following example, the [^mo^] element
               is used for the binary operator +. Default spacing is symmetric
               around that operator. A tighter spacing is used if you rely
-              on the <a><code>form</code></a> attribute to force it to be
+              on the [^mo/form^] attribute to force it to be
               treated as a prefix operator.
               Spacing can also be specified explicitly using the
               [^mo/lspace^] and