mention the 'Alt' operator as another one that remains after the translation of property path patterns

Author: hartig

spec/index.html

@@ -9807,6 +9807,7 @@ <h3>Property Path Patterns</h3>
           to other SPARQL graph patterns, such as converting property paths of length one to triple
           patterns, which in turn are combined into basic graph patterns.
           This leaves algebraic property path expressions with the operators
+          <a href="#defn_ppeAlt" class="ppeOp">Alt</a>,
           <a href="#defn_ppeZeroOrOnePath" class="ppeOp">ZeroOrOnePath</a>,
           <a href="#defn_ppeZeroOrMorePath" class="ppeOp">ZeroOrMorePath</a>,
           <a href="#defn_ppeOneOrMorePath" class="ppeOp">OneOrMorePath</a>,