KUnit: Docs: usage: wording fixes

Fix minor grammar and punctutation glitches.
Hyphenate "architecture-specific" instances.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: David Gow <davidgow@google.com>
Cc: linux-kselftest@vger.kernel.org
Cc: kunit-dev@googlegroups.com
Cc: Shuah Khan <shuah@kernel.org>
Cc: Shuah Khan <skhan@linuxfoundation.org>
Cc: Brendan Higgins <brendanhiggins@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>