[readme] add examples of setting specific versions as default

Co-authored-by: heungjun.park <rockheung@gmail.com>
Co-authored-by: Jordan Harband <ljharb@gmail.com>

README.md

@@ -499,7 +499,9 @@ nvm deactivate
 To set a default Node version to be used in any new shell, use the alias 'default':
 
 ```sh
-nvm alias default node
+nvm alias default node # this refers to the latest installed version of node
+nvm alias default 18 # this refers to the latest installed v18.x version of node
+nvm alias default 18.12  # this refers to the latest installed v18.12.x version of node
 ```
 
 #### Use a mirror of node binaries