kbuild: [PATCH] document modules_install in "make help"

The patch below adds information about modules_install to "make help".

Makefile

@@ -799,7 +799,8 @@ help:
 	@echo  '  all		  - Build all targets marked with [*]'
 	@echo  '* vmlinux	  - Build the bare kernel'
 	@echo  '* modules	  - Build all modules'
-	@echo  '  dir/file.[ois]- Build specified target only'
+	@echo  '  modules_install - Install all modules'
+	@echo  '  dir/file.[ois]  - Build specified target only'
 	@echo  '  rpm		  - Build a kernel as an RPM package'
 	@echo  '  tags/TAGS	  - Generate tags file for editors'
 	@echo  ''