fix for signature help

[svenefftinge]

It should be parameters not params.
(see https://github.com/Microsoft/language-server-protocol/blob/master/protocol.md#textDocument_signatureHelp)

[palantirtech]

Thanks for your interest in palantir/python-language-server, @svenefftinge! Before we can accept your pull request, you need to sign our contributor license agreement - just visit https://cla.palantir.com/ and follow the instructions. Once you sign, I'll automatically update this pull request.

[gatesn]

Good catch, thanks!

[gatesn]

@svenefftinge would love to get this merged if you could sign the CLA, thanks!