fix: signature help now highlights the right parameter

[grecodavide]

Currently, signature help does not highlight the current parameter but always starts from the second character (see image)

By using the right indexes, this gets fixed:

[Saghen]

What version of neovim are you on? I'm unable to reproduce this on nvim 0.10.2

[grecodavide]

I am on v0.11.0-dev-1106+gf7e32fb6e6

EDIT:
To provide additional information, I checked convert_signature_help_to_markdown_lines in vim/lsp/util.lua and can confirm that it returns a table like this:

{start_row, start_col, end_row, end_col}

[grecodavide]

With this commit the hl should work as expected in both versions

[Saghen]

Thanks!