doc: fix the version for a chapter

This is the KMail version (KDE is not a product).

CCMAIL: davidbryant@gvtc.com

doc/kmail2/using-kmail.docbook

@@ -18,7 +18,7 @@
 </authorgroup>
 
 <date>2020-09-12</date>
-<releaseinfo>KDE 5.14.2 (Applications 20.04.2)</releaseinfo>
+<releaseinfo>5.14.2 (Applications 20.04.2)</releaseinfo>
 </chapterinfo>
 
 <title>Using &kmail;</title>