fixup! Rename premise in index_guard in string constraint

Author: romainbrenguier

src/solvers/refinement/string_constraint.h

@@ -56,8 +56,8 @@ Author: Romain Brenguier, romain.brenguier@diffblue.com
 class string_constraintt final
 {
 public:
-  // String constraints are of the form
-  // forall univ_var in [lower_bound,upper_bound[. premise => body
+  /// String constraints are of the form
+  /// forall univ_var in [lower_bound,upper_bound[. index_guard => body
   exprt index_guard = true_exprt(); // Index guard
   exprt body;                       // value constraint
   symbol_exprt univ_var;