docs: more accurate typing for LSP references context

**Problem:** The `context` parameter for `references()` is just typed as
a table, which is unhelpful.

**Solution:** Properly type it as an `lsp.ReferenceContext`!

runtime/lua/vim/lsp/buf.lua

@@ -736,7 +736,7 @@ end
 
 --- Lists all the references to the symbol under the cursor in the quickfix window.
 ---
----@param context (table|nil) Context for the request
+---@param context lsp.ReferenceContext? Context for the request
 ---@see https://microsoft.github.io/language-server-protocol/specifications/specification-current/#textDocument_references
 ---@param opts? vim.lsp.ListOpts
 function M.references(context, opts)