linux appdata: Update date in release notes

Author: hbons

SparkleShare/Linux/org.sparkleshare.SparkleShare.appdata.xml

@@ -14,7 +14,7 @@
     </description>
 
     <releases>
-        <release version="3.28.0" date="2018-03-18" urgency="medium">
+        <release version="3.28.0" date="2018-07-14" urgency="medium">
             <description>
                 <p>Fixes and improvements:</p>
                 <ul>