Fix hover on method parameters with inline RBS annotations

[whtsht]

There was an issue where hovering was not possible when method arguments had rbs comments.

The root cause was an off-by-one error: Prism::Location#start_line is 1-based, but rbs_comments is indexed by 0-based line numbers.
As a result, the first element of rbs_comments ended up being nil, e.g.:

[nil, " (Integer) -> void"]

whtsht@0e6baab

This PR fixes the issue by subtracting 1 from start_line before indexing/storing into rbs_comments.

[whtsht]

screenshot :)

before

after