docs: fix example for within function

[mdrobny]

I struggled with switching Node.js version via nvm in my zx script because docs are misleading currently - nvm use 16 must be part of prefix to allow using Node.js 16 inside the context

I found fix was provided in #362 but docs are incorrect

• Appropriate changes to README are included in PR

---

Example

Current version

await $`node --version`

let version = await within(async () => {
    $.prefix += 'export NVM_DIR=$HOME/.nvm; source $NVM_DIR/nvm.sh;'

    await $`nvm use 16`

    return $`node --version`
})

echo(version);

$ node --version
v20.2.0
$ nvm use 16 <--- switching to version 16
Now using node v16.20.0 (npm v8.19.4)
$ node --version
v20.2.0 <---- but still 20 is used
v20.2.0

Fixed version

await $`node --version`

let version = await within(async () => {
    $.prefix += 'export NVM_DIR=$HOME/.nvm; source $NVM_DIR/nvm.sh; nvm use 16;'

    return $`node --version`
})

echo(version);

$ node --version
v20.2.0
$ node --version
Now using node v16.20.0 (npm v8.19.4)
v16.20.0 <---- it's actually 16 here
Now using node v16.20.0 (npm v8.19.4)
v16.20.0

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[antonmedv]

Thanks!