Add missing period at the end of sentence

[mine-cetinkaya-rundel]

The error currently reads something like the following:

Error: `data` must be a data frame, or other object coercible by `fortify()`, not an S3 object with class uneval
Did you accidentally pass `aes()` to the `data` argument?

This PR adds a period at the end of the first sentence.

[clauswilke]

/document

[clauswilke]

For a moment I thought this needed updated documentation files, but of course it doesn't.

[yutannihilation]

The failure on R-devel seems irrelevant to this pull request (I'll fix later). Can we merge this?

Graphics API version mismatch

[clauswilke]

I think so. I was just running out the door when I approved this so it fell through the cracks.

[yutannihilation]

Thanks!