Makefile: kselftest: fix grammar typo

	@echo  '                    Build, install, and boot kernel before'

	@echo  '                    running kselftest on it'

	@echo  '  kselftest-clean - Remove all generated kselftest files'

	@echo  '  kselftest-merge - Merge all the config dependencies of kselftest to existed'

	@echo  '  kselftest-merge - Merge all the config dependencies of kselftest to existing'

	@echo  '                    .config.'

	@echo  ''

	@echo 'Userspace tools targets:'