[readme] add examples of setting specific versions as default

[Rockheung]

In numerous blog posts regarding the configuration of the default nodejs version using nvm, the following command is often cited:

nvm alias default node

While this command might work in some cases (as node will point to the latest version installed on your system), it might not behave as intended if you want to default to a specific version of Node.js.

This command is actually referring to the Set default node version section in the nvm's README. It's crucial to note that the instructions suggest setting a specific node version of your choosing, rather than using the generic node.

So if you're aiming to set a particular version (for example, 14.15.1) as your default, the correct command would be:

nvm alias default 14.15.1

This ensures that the specified version of Node.js will be used by default. If you simply use node as in the first command, it could point to any installed version, which might not be what you intend.

[Rockheung]

This PR's base branch outdated, so closing

[ljharb]

that's not a reason to close a PR; that's a reason to rebase it.

[ljharb]

I tweaked your suggestion to be a bit more concise while hopefully being helpful.