Doc: update screenshot for external tools

Change-Id: I3bdb4fd40c12efbd5d40eccb1f636748ec3aa501
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@nokia.com>

doc/src/howto/creator-external-tools.qdoc

@@ -97,7 +97,7 @@
     additional tools in \QC \gui Options.
 
     You can use \QC variables in the fields that you can select from
-    lists of available \QC variables.
+    lists of available \QC variables (1).
 
     \image qtcreator-external-tools.png "External Tools options"