docs: use "ev" convention in event-handlers

Problem:
In autocmd examples, using "args" as the event-object name is vague and
may be confused with a user-command.

Solution:
Use "ev" as the conventional event-object name.

runtime/lua/vim/treesitter.lua

@@ -438,9 +438,9 @@ end
 ---
 --- ```lua
 --- vim.api.nvim_create_autocmd( 'FileType', { pattern = 'tex',
----     callback = function(args)
----         vim.treesitter.start(args.buf, 'latex')
----         vim.bo[args.buf].syntax = 'ON'  -- only if additional legacy syntax is needed
+---     callback = function(ev)
+---         vim.treesitter.start(ev.buf, 'latex')
+---         vim.bo[ev.buf].syntax = 'ON'  -- only if additional legacy syntax is needed
 ---     end
 --- })
 --- ```