chore(examples): fix color binding for progress-spinner (#4687)

* Replaces the invalid `attr.color` with a property binding. This fixes the progress-spinner demo on the docs.

Fixes #4617

Author: devversion

src/material-examples/progress-spinner-configurable/progress-spinner-configurable-example.html

@@ -41,7 +41,7 @@ <h2 class="example-h2">Result</h2>
 
     <md-progress-spinner
         class="example-margin"
-        [attr.color]="color"
+        [color]="color"
         [mode]="mode"
         [value]="value">
     </md-progress-spinner>