kbuild: "make prepare" should be "make modules_prepare"

Correct the Makefile help text to read "make modules_prepare".
Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

Makefile

@@ -1293,7 +1293,7 @@ help:
 	@echo  '  dir/            - Build all files in dir and below'
 	@echo  '  dir/file.[ois]  - Build specified target only'
 	@echo  '  dir/file.ko     - Build module including final link'
-	@echo  '  prepare         - Set up for building external modules'
+	@echo  '  modules_prepare - Set up for building external modules'
 	@echo  '  tags/TAGS	  - Generate tags file for editors'
 	@echo  '  cscope	  - Generate cscope index'
 	@echo  '  kernelrelease	  - Output the release version string'