Change 'usecase' to 'use case'.

Why is this change being made?

'Usecase' should be 'use case'. It's possible this change could break inbound links, so that needs to be checked.

Does this MR meet the acceptance criteria?

Assign to DRI

• Did you assign this change to the correct DRI of the page or information you are changing?

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what