debian/changelog: fix trailing whitespace at line 641 to appease Lintian

debian/changelog

@@ -638,7 +638,7 @@ mumble (1.2.3-349-g315b5f5-2.1) unstable; urgency=low
          - fix murmur.ini example for opusthreshold option to =100
            (instead of =1)
          - add #opusthreshold=100 comments to murmur.ini.system (shipped as
-           etc/mumble-server.ini) to match murmur.ini example    
+           etc/mumble-server.ini) to match murmur.ini example
     Closes: #675971.
     This implements the TC decision in #682010.
     [Thanks to Gregor Herrmann for his help and guidance.]