Add an example using foldMap on to a tuple

[philwills]

This is based on feedback from @adelbertc to a previous PR.

I've also changed 'typeclass' to 'type class'.

[codecov-io]

Current coverage is 63.79%

Merging #517 into master will not affect coverage as of 504c8e8

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  Missed         854     854       

Review entire Coverage Diff as of 504c8e8

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[adelbertc]

Hurrah! We currently don't have Monoid instances for TupleN (we need to fix that..) but until then this LGTM 👍

Following https://github.com/non/cats/blob/3c2c516676d6db733b08ee9d00396df52b90e94a/PROCESS.md#merging-pull-requests I'm merging this