ExternalProject: Fix error message typo

Modules/ExternalProject.cmake

@@ -1553,7 +1553,7 @@ function(ExternalProject_Add_StepDependencies name step)
   if(TARGET ${name}-${step})
     get_property(is_ep_step TARGET ${name}-${step} PROPERTY _EP_IS_EXTERNAL_PROJECT_STEP)
     if(NOT is_ep_step)
-      message(FATAL_ERROR "Target \"${name}\" was not generated by ExternalProject_Add_StepTargets.")
+      message(FATAL_ERROR "Target \"${name}-${step}\" was not generated by ExternalProject_Add_StepTargets.")
     endif()
   endif()