Help: Update example Qt 5 find_package call to add missing component

Running CMake on it caused the following error:

    error: Target "publisher" links to target "Qt5::DBus" but the target
    was not found.  Perhaps a find_package() call is missing for an
    IMPORTED target, or an ALIAS target is missing?

Add the missing DBus component.

Help/manual/cmake-qt.7.rst

@@ -29,7 +29,7 @@ Qt 4 and Qt 5 may be used together in the same
   set(CMAKE_AUTOMOC ON)
   set(CMAKE_INCLUDE_CURRENT_DIR ON)
 
-  find_package(Qt5 COMPONENTS Widgets REQUIRED)
+  find_package(Qt5 COMPONENTS Widgets DBus REQUIRED)
   add_executable(publisher publisher.cpp)
   target_link_libraries(publisher Qt5::Widgets Qt5::DBus)