fix placement type in PopperProps.children

[JLarky]

type is undefined | PopperJS.Placement, not ?PopperJS.Placement since that doesn't mean anything in typescript and was probably copied from Flow

fixes error that I get in typescript compiler:

node_modules/react-popper/typings/react-popper.d.ts
(24,16): JSDoc types can only be used inside documentation comments.

[FezVrasta]

Thanks for the PR, we do have a test to verify that the ts types are right. May you check why isn't it detecting the problem please?

[JLarky]

I was wondering about that myself. Maybe it has to do with typescript version. But since this was clearly the fix I wasn't going to investigate it too much.

[JLarky]

@FezVrasta I decided to test it :)

so it has nothing to do with typescript version, just with config option that was suppressing this warning in your CI tests

[JLarky]

And I guess just to be clear what this PR does:

[JLarky]

@FezVrasta do you need anything else for this PR?

[giladgray]

this PR fixes #152!
i also just opened #153 with a more weighty refactor to the types to make them more useful.

[JLarky]

@giladgray I'm just waiting for any of them to be merged so I can start using popper inside my project :) but I guess one more PR is needed that would remove skipLibCheck that was underlying issue that made #152 possible

[FezVrasta]

Thanks for looking into it, may you adjust the TS configuration so that it correctly detects this problem in the future? We can do everything in this PR