lang file: missing keyword.

share/lang/why3.lang

@@ -170,6 +170,7 @@ on linking described in file LICENSE.
           <keyword>import</keyword>
           <keyword>inductive</keyword>
           <keyword>lemma</keyword>
+          <keyword>mutable</keyword>
           <keyword>module</keyword>
           <keyword>namespace</keyword>
           <keyword>predicate</keyword>