Documentation: kunit: fix typos and gramatical errors

Fix typos and gramatical errors in the Getting Started and Usage guide
for KUnit.

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://patchwork.kernel.org/patch/11156481/


Reported-by: Rinat Ibragimov <ibragimovrinat@mail.ru>
Link: https://github.com/google/kunit-docs/issues/1


Signed-off-by: Brendan Higgins <brendanhiggins@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>