Fix documentation for every_n_epochs parameter of ModelCheckpoint

[adamreeve]

What does this PR do?

Fixes documentation on the every_n_epochs parameter in ModelCheckpoint. This incorrectly states that leaving the value as the default of None means that checkpoints are not saved.

Does your PR introduce any breaking changes? If yes, please list them.

No

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[tchaton]

Looks great !

[adamreeve]

Do I need to do anything to get this merged? Looks like it's blocked on some test failures that look unrelated to this change, plus not having a linked issue, although from the PR template this shouldn't be required for a docs change.