doc: fix trailing whitespace in Doxygen XML

Author: LudwigKnuepfer

doc/doxygen/RIOTDoxygenLayout.xml

@@ -11,15 +11,15 @@
     </tab>
     <tab type="classes" visible="yes" title="">
       <tab type="classlist" visible="yes" title="" intro=""/>
-      <tab type="classindex" visible="$ALPHABETICAL_INDEX" title=""/> 
+      <tab type="classindex" visible="$ALPHABETICAL_INDEX" title=""/>
       <tab type="hierarchy" visible="yes" title="" intro=""/>
       <tab type="classmembers" visible="yes" title="" intro=""/>
     </tab>
     <tab type="files" visible="yes" title="">
       <tab type="filelist" visible="yes" title="" intro=""/>
       <tab type="globals" visible="yes" title="" intro=""/>
     </tab>
-    <tab type="examples" visible="yes" title="" intro=""/>  
+    <tab type="examples" visible="yes" title="" intro=""/>
   </navindex>
 
   <!-- Layout definition for a class page -->