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.

examples/maxsat/maxsat.c

@@ -15,7 +15,7 @@ Copyright (c) 2015 Microsoft Corporation
 /** 
    \defgroup maxsat_ex MaxSAT/MaxSMT examples
 */
-/*@{*/
+/**@{*/
 
 /**
    \brief Exit gracefully in case of error.
@@ -638,5 +638,5 @@ int main(int argc, char * argv[]) {
     return 0;
 }
 
-/*@}*/
+/**@}*/