Update README about method definition support

[sathishmanohar]

I was under the impression that jump to definitions feature isn't available based on the README. I found out that there were recent improvements in this regard.

https://github.com/Shopify/ruby-lsp/releases/tag/v0.16.5

So, made an edit to remove the part in README indicating that jump to definitions feature isn't working.

[sathishmanohar]

I have signed the CLA!

[andyw8]

Thanks!