Update property syntax in README.md

[MarkJamesHoward]

In the Declarative Rendering section of README.md it seems to be using lit-html syntax rather than how we use properties in LitElement where we'd need to specify the 'this' keyword?

So rather than:

class="${color} special"

we should have

class="${this.color} special"

Reference Issue

Fixes #197

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[MarkJamesHoward]

CLA signed

[sorvell]

Thanks for the PR. Can you please verify that the CLA is signed? The bot is still complaining.

[MarkJamesHoward]

Ah sorry different email on my google account and Github - I've used the Github email to sign the CLA now. Hopefully it goes green :-)

[googlebot]

CLAs look good, thanks!