doc: fix keyObject.symmetricSize to be keyObject.symmetricKeySize

This PR fixes wrong property in crypto documentation for an API added in 11.6.0

cc @tniessen

k = crypto.createSecretKey(Buffer.from('foo'))
// SecretKeyObject { [Symbol(kKeyType)]: 'secret' }
k.symmetricSize
// undefined
k.symmetricKeySize
// 3

Checklist

• documentation is changed or added
• commit message follows commit guidelines