Add missing colon to part forwarding example.

As noted in the [fix for the explainer doc](fergald/docs#2), this missing colon is probably an oversight after the syntax change.

Author: oslego

css-shadow-parts-1/Overview.bs

@@ -218,7 +218,7 @@ Note: It's okay to map a sub-part to several names.
 </style>
 
 <template id="c-e-outer-template">
-  &lt;c-e-inner <b>exportparts="innerspan textspan"</b>&gt;&lt;/c-e-inner&gt;
+  &lt;c-e-inner <b>exportparts="innerspan: textspan"</b>&gt;&lt;/c-e-inner&gt;
 &lt;/template&gt;
 
 &lt;template id="c-e-inner-template"&gt;