docs: update logger section in README (zbirenbaum#397)

Incorrect example in the README and addition of more information in the logger section

Author: AntoineGS

README.md

@@ -88,10 +88,8 @@ require('copilot').setup({
     ["."] = false,
   },
   logger = {
-    log_to_file = false,
     file = vim.fn.stdpath("log") .. "/copilot-lua.log",
-    file_log_level = vim.log.levels.WARN,
-    print_log = true,
+    file_log_level = vim.log.levels.OFF,
     print_log_level = vim.log.levels.WARN,
     trace_lsp = "off", -- "off" | "messages" | "verbose"
     trace_lsp_progress = false,
@@ -224,6 +222,7 @@ require("copilot").setup {
 
 Logs will be written to the `file` for anything of `file_log_level` or higher.
 Logs will be printed to NeoVim (using `notify`) for anything of `print_log_level` or higher.
+To turn either off, simply set its level to `vim.log.levels.OFF`.
 File logging is done asynchronously to minimize performance impacts, however there is still some overhead.
 
 Log levels used are the ones defined in `vim.log`: