Hook screens text and capitalization

[apinnick]

I noticed some places where title case was used instead of sentence case. The other UI screens use sentence case, so it's good to be consistent.

https://github.com/konveyor/mig-operator/blob/master/docs/usage/screenshots/hooks/add-hook.png

Change "Add Hook" to add "Add hook". (See "Add migration plan" on the Migration plans screen.)

https://github.com/konveyor/mig-operator/blob/master/docs/usage/screenshots/hooks/hook-name-playbook.png

Hook Name > Hook name
Hook Definition > Hook definition