docs(lua): iInconsistent vim.keymap param name #37026

Problem: vim.keymap.del has 'modes' as it's first argument while vim.keymap.set
has 'mode' as it's first argument despite both 'mode' and 'modes' taking in the
same type input of String or String[].

Solution: Updated vim.keymap.set docs to refer to it's first argument
as 'modes'.

test/functional/lua/vim_spec.lua

@@ -3042,7 +3042,7 @@ describe('vim.keymap', function()
 
   it('validates', function()
     matches(
-      'mode: expected string|table, got number',
+      'modes: expected string|table, got number',
       pcall_err(exec_lua, [[vim.keymap.set(42, 'x', print)]])
     )