Documentation/dev-tools: clean up kselftest.rst

This is a small cleanup to kselftest.rst:

- Fix some language typos in the usage instructions.
- Change one non-ASCII space to an ASCII space.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Shuah Khan <shuah@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>