[css-values-3] Add link default for color definition

Without this fix I was getting the following error from bikeshed:

```
LINK ERROR: Multiple possible 'color' maybe refs.
Arbitrarily chose
https://drafts.csswg.org/css-values-4/#valdef-type-or-value-color
To auto-select one of the following refs, insert one of these lines into
a <pre class=link-defaults> block:
spec:compositing-2; type:value; text:color
spec:css-values-4; type:value; text:color
```

I guess compositing-2 added a definition for color.

Author: birtles

css-values-3/Overview.bs

@@ -29,6 +29,7 @@ Inline Github Issues: yes
 spec: css-backgrounds-3; type: type; text: <position>
 spec: css21; type: property; text: border-collapse
 spec: css-color-4; type: value; text: currentcolor
+spec: css-color-4; type: value; text: color
 spec: css-cascade-4; type: at-rule; text: @import
 </pre>
 <style>