Fix substitution mapping

* Update index.html

small fix in substitution

* Update spec/index.html

Co-authored-by: Ted Thibodeau Jr <[email protected]>

---------

Co-authored-by: Ted Thibodeau Jr <[email protected]>

Author: doerthe

spec/index.html

@@ -249,7 +249,7 @@ <h2>Notation and Terminology</h2>
     <p>A <dfn>ground</dfn> RDF graph is one that contains no blank nodes.</p>
 
   <p>
-    For RDF terms t, x, and y, we define the <dfn>substitution mapping</dfn> t[x/y] inductively, as follows:
+    For RDF terms t, x, and y, where either y is not a triple term or x does not appear in y, we define the <dfn>substitution mapping</dfn> t[x/y] inductively, as follows:
   </p>
   <ol>
     <li>If t = x, then t[x/y] = y.</li>