fix(common-questions): use branched accessors in legend (discordjs#459)

Author: louisglen1

guide/popular-topics/common-questions.md

@@ -4,7 +4,7 @@
 
 * `<client>` is a placeholder for the Client object, such as `const client = new Discord.Client();`.
 * `<message>` is a placeholder for the Message object, such as `client.on('message', message => { ... });`.
-* `<guild>` is a placeholder for the Guild object, such as `<message>.guild` or `<client>.guilds.get('<id>')`.
+* `<guild>` is a placeholder for the Guild object, such as `<message>.guild` or <branch version="12.x" inline>`<client>.guilds.cache.get('<id>')`</branch><branch version="11.x" inline>`<client>.guilds.get('<id>')`</branch>.
 
 For a more detailed explanation on the notations commonly used in this guide, the docs, and the support server, see [here](/additional-info/notation.md).