cob: fix documentation — heartwood

no_run will compile but not run the code. Use ignore instead, and also added rust for syntax highlighting.

 ///
 /// # Usage
 ///
 /// ```no_run
 /// ```rust, ignore
 /// let oid1 = with_advanced_timestamp(|| cob.update("New revision OID"));
 /// let oid2 = with_advanced_timestamp(|| cob.update("Another revision OID"));
 /// ```