Undeclared foo prefix: bar

All namespace prefixes must be declared prior to use. Namespace prefixes are case-sensitive.

Correct the typo, or declare the prefix with an xmlns: attribute, for example:

<Url type="application/rss+xml" 
     xmlns:example="http://example.com/opensearchextensions/1.0/"
     template="http://example.com?f={example:format?}"/>