Fix Doxygen formatting in parse_options.h

The use of `\example` in this file created a new section in the main
(side bar) menu.  In order to limit the categories in the top level
menu, the formatting is changed here to be more like how examples are
given in other sections of our documentation.

Also, a full stop is used to separate a method summary from more
detailed documentation, to make the generated documentation look better.

Author: John Nonweiler

src/util/parse_options.h

@@ -45,12 +45,13 @@ class parse_options_baset
 std::string
 banner_string(const std::string &front_end, const std::string &version);
 
-/// Utility for displaying help centered messages borderered by "* *"
-/// - we use this for displaying banner information and the like
-/// in help messages
-/// \example
+/// Utility for displaying help centered messages borderered by "* *".
+/// We use this for displaying banner information and the like
+/// in help messages.
+/// ```
 ///   align_center_with_border("test-text")
 ///   == "* *                        test-text                        * *"
+/// ```
 std::string align_center_with_border(const std::string &text);
 
 #endif // CPROVER_UTIL_PARSE_OPTIONS_H