fix grouping for latest doxygen (#5626)

Since doxygen 1.8.16, opening and closing a group must not be done as
C comment but as doxygen command. In other words, not one but two
asterisk characters are required so that doxygen finds a group.

src/api/z3_optimization.h

@@ -28,10 +28,10 @@ extern "C" {
 #endif // __cplusplus
 
     /** \defgroup capi C API */
-    /*@{*/
+    /**@{*/
 
     /** @name Optimization facilities */
-    /*@{*/
+    /**@{*/
     /**
        \brief Create a new optimize context.
 
@@ -368,8 +368,8 @@ extern "C" {
         Z3_model_eh  model_eh);
 
 
-    /*@}*/
-    /*@}*/
+    /**@}*/
+    /**@}*/
 
 #ifdef __cplusplus
 }