Refs #12722 Also cover range summation

We may as wel make sure to cover *all* the syntax in the doc test, to
ensure it all works.

Code/Mantid/docs/source/algorithms/GroupDetectors-v2.rst

@@ -491,22 +491,24 @@ Output:
    # Create Workspace of 10 spectra each with one bin.
    ws = CreateWorkspace(DataX=[1], DataY=[1,2,3,4,5,6,7,8,9,10], NSpec=10)
 
-   # Run algorithm adding first two spectra and then keep the fifth, and last three spectra
-   ws2 = GroupDetectors(ws, GroupingPattern="0+1,4,7:9")
+   # Run algorithm adding first two spectra and then keep the fourth, add the fifth to seventh, and then keep the last three spectra
+   ws2 = GroupDetectors(ws, GroupingPattern="0+1,3,4-6,7:9")
 
    #print result
    print ws2.readY(0)
    print ws2.readY(1)
    print ws2.readY(2)
    print ws2.readY(3)
    print ws2.readY(4)
+   print ws2.readY(5)
 
 Output:
 
 .. testoutput:: ExGroupDetectorsWithPattern
 
    [ 3.]
-   [ 5.]
+   [ 4.]
+   [ 18.]
    [ 8.]
    [ 9.]
    [ 10.]