snippet: Updated all snippet tags

Ensure all src C++ headers which have corresponding C++ examples have
doxygen tags which point to those examples.  Some were missing, some
were invalid, some needed to be updated to match the new example names.

Signed-off-by: Noel Eck <noel.eck@intel.com>

src/gas/mq6.hpp

@@ -47,7 +47,7 @@ namespace upm {
    * at least 24 hours.  See the datasheet for details.
    *
    * @image html mq2-5.jpeg
-   * @snippet mq6.cxx Interesting
+   * @snippet gas-mq6.cxx Interesting
    */
     class MQ6 : public Gas {
         public: