Update default encoding for feature.xml to UTF-8

The feature.xml file has migrated from "ISO-8859-1" to "UTF-8" as
default encoding a long time ago. This updates the documentation to
reflect this change.

Author: ptziegler

eclipse.platform.common/bundles/org.eclipse.platform.doc.isv/reference/misc/feature_manifest.html

@@ -15,7 +15,7 @@ <h1>
 <p><i>Version 4.2 - Last revised May, 2012</i></p>
 
 <p>The feature manifest format is defined by the following dtd:</p>
-<p><tt>&lt;?xml encoding="ISO-8859-1"?&gt;</tt></p>
+<p><tt>&lt;?xml encoding="UTF-8"?&gt;</tt></p>
 <p><tt>&lt;!ELEMENT feature (install-handler? , description? , copyright? ,
 license? , url? , includes* , requires? , plugin* , data*)&gt;</tt>
 <br><tt>&lt;!ATTLIST feature</tt>