Update UsingNavigators.md

[ethangk]

Added a note about components not importing if their format is.jsx. I couldn't find this mentioned anywhere in the documentation and it had me scratching my head for a little while.

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[hramos]

This doc no longer exists in master as of 08dbc43. Avoid opening pull requests against release branches, as those are built from commits in master.

[ethangk]

Ok thanks for the heads up