Correct the indexes SANSadd2.py

The event data has the monitors in its head. Now, it copies the corresponded
values.

re #7600
(cherry picked from commit 7884ac7)

Code/Mantid/scripts/SANS/SANSadd2.py

@@ -117,9 +117,9 @@ def add_runs(runs, inst='sans2d', defType='.nxs', rawTypes=('.raw', '.s*', 'add'
             wsOut.setY(i,wsInMonitor.dataY(i))
             wsOut.setE(i,wsInMonitor.dataE(i))               
 
-        for i in range(wsOut.getNumberHistograms() - mon_n):
-            wsOut.setY(i+mon_n, wsInDetector.dataY(i))
-            wsOut.setE(i+mon_n, wsInDetector.dataE(i))
+        for i in range(mon_n, wsOut.getNumberHistograms()):
+            wsOut.setY(i, wsInDetector.dataY(i))
+            wsOut.setE(i, wsInDetector.dataE(i))
 
         if 'AddFilesSumTempory_Rebin' in mtd : DeleteWorkspace('AddFilesSumTempory_Rebin')