Switch to passing PartialObserver<T> to subscribe

[MatthewAlner]

The current use of subscribe shows a deprecation warning in newer versions of Angular

[MatthewAlner]

Hey!

Thanks for the article it was just what I needed you saved me some time.

There were a couple of things I saw that needed updating while I was copy-pasting from the article's code snippets into my project.

The return types in the netlify-forms.service needed a type : Observable<string> but I see that is already fixed in this repo but not on the site.

Then there was also a deprecation warning in my Angular project I guess because it's newer with a newer rxjs for the subscribe method. This small PR updates that.

Thanks Again

Matt