Update contact-manager-tutorial.md
Update contact-manager-tutorial.md

My message to Rob:
Hi Rob,
I have found a couple of typos (not so important but maybe nice to fix).
Also, I have seen some differences in the use of 'this' prefix in 'contact-detail' files.
I am not 100% sure that the proposed change is correct, but it works on my machine. :)
Feel free to accepts some or even none of the changes.
I have fallen in love with your Aurelia but I have yet too far to go.
Maybe these minor changes will prove to be useful.
Best regards,
Andre
aurelia/framework,aurelia/framework