Documentation: fix the case of @tparam Mutex

This fixes the following Clang warning:
.../kdevelop/kdevplatform/serialization/itemrepository.h:1083:12: warning: template parameter 'mutex' not found in the template declaration [-Wdocumentation]
 * @tparam mutex The mutex type to use internally. It has to be locked externally before accessing the item repository
           ^~~~~
.../kdevelop/kdevplatform/serialization/itemrepository.h:1083:12: note: did you mean 'Mutex'?
 * @tparam mutex The mutex type to use internally. It has to be locked externally before accessing the item repository
           ^~~~~
           Mutex