use 'registerChild' as ref in registerChild example

[pinko-fowle]

In the registerChild example in CellMeasurer, we never use registerChild. From looking around some, I believe one valid use is to pass it as ref={registerChild}, which I do here.

I'm still quite unsure of this PR but it seems to match most of the examples I've found. And the current state of the docs seems quite weird/confusing, as we introduce this param then don't use it. I hope this PR makes sense, & please forgive me if not.

One example of another user here following this pattern is this comment: #1572 (comment)

• For any documentation changes, the text has been proofread and is clear to both experienced users and beginners.
• Format your code with prettier (yarn run prettier).