fix: formatter for html

Fixes error caused by html formatter running even when tidy is not
available on the system. Also fixes docs related to disabling
formatters.

README.md

@@ -80,7 +80,7 @@ use {
         show_http_info = true,
         show_headers = true,
         -- executables or functions for formatting response body [optional]
-        -- set them to nil if you want to disable them
+        -- set them to false if you want to disable them
         formatters = {
           json = "jq",
           html = function(body)

lua/rest-nvim/config/init.lua

@@ -16,6 +16,9 @@ local config = {
     formatters = {
       json = "jq",
       html = function(body)
+        if vim.fn.executable("tidy") == 0 then
+          return body
+        end
         -- stylua: ignore
         return vim.fn.system({
           "tidy", "-i", "-q",