DOC: flag only uses one dash

CMake/OTBModuleEnablement.cmake

@@ -202,7 +202,7 @@ foreach(otb-module ${OTB_MODULES_ALL})
   if(${otb-module}_ENABLED)
     # check cxx11 requirement after all enable/disable macros have been passed
     if(OTB_MODULE_${otb-module}_REQUIRES_CXX11 AND NOT OTB_HAS_CXX11)
-      message(FATAL_ERROR "Module ${otb-module} requires C++11 support. Consider adding --std=c++11 to your compiler flags or disabling it.")
+      message(FATAL_ERROR "Module ${otb-module} requires C++11 support. Consider adding -std=c++11 to your compiler flags or disabling it.")
     endif()
 
     list(APPEND OTB_MODULES_ENABLED ${otb-module})