FindDoxygen: Use gerund form "Generating" instead of "Generate"

This aligns the default comment produced by doxygen_add_docs() a bit
with the other CMake comments used at build phase.

Modules/FindDoxygen.cmake

@@ -874,7 +874,7 @@ function(doxygen_add_docs targetName)
                           ${ARGN})
 
     if(NOT _args_COMMENT)
-        set(_args_COMMENT "Generate API documentation for ${targetName}")
+        set(_args_COMMENT "Generating API documentation for ${targetName}")
     endif()
 
     if(NOT _args_WORKING_DIRECTORY)