Bump suggested eslint config's ecma version (discordjs#245)

Author: tipakA

guide/preparations/setting-up-a-linter.md

@@ -49,7 +49,7 @@ ESLint may display a lot of warnings and errors about your code when you start u
 "es6": true
 },
 "parserOptions": {
-"ecmaVersion": 2017
+"ecmaVersion": 2019
 },
 "rules": {
 
@@ -71,7 +71,7 @@ Alternatively, if you don't want to go through everything one-by-one on your own
 "es6": true
 },
 "parserOptions": {
-"ecmaVersion": 2017
+"ecmaVersion": 2019
 },
 "rules": {
 "brace-style": ["error", "stroustrup", { "allowSingleLine": true }],