fix: use correct MultiSegmentation syntax (#28)

### Briefly, what does this PR introduce?
#26 introduced a regression due to incorrect MultiSegmentation syntax.
This fixes the syntax to ensure downstream benchmarks can run again.

### What kind of change does this PR introduce?
- [X] Bug fix (issue #__)
- [ ] New feature (issue #__)
- [ ] Documentation update
- [ ] Other: __

### Please check if this PR fulfills the following:
- [ ] Tests for the changes have been added
- [ ] Documentation has been added / updated
- [X] Changes have been communicated to collaborators @shimasnd
@ajentsch @veprbl (but I won't wait for a review due to tomorrow's
deadline)

### Does this PR introduce breaking changes? What changes might users
need to make to their code?
No.

### Does this PR change default behavior?
Yes; default will work again.

ip6/far_forward/ZDC_WSi.xml

@@ -2,9 +2,9 @@
   <comment>
     //////////////////////////////////////////////////
     // Far Forward Zero Degree Calorimeter
-    // Crystal calorimeter 
+    // Crystal calorimeter
     //////////////////////////////////////////////////
-  </comment>        
+  </comment>
 
   <define>
     <constant name="ZDC_WSi_x"       value="ZDC_width"/>
@@ -68,12 +68,13 @@
 
   <readouts>
     <readout name="ZDC_WSi_Hits">
-      <segmentation type="MultiSegmentation" key="silicon"/>
-      <segmentation name="WSi_Pixel1" type="CartesianGridXY" key_value="1" grid_size_x="3.*mm" grid_size_y="3.*mm"/>
-      <segmentation name="WSi_Pixel2" type="CartesianGridXY" key_value="12" grid_size_x="3.*mm" grid_size_y="3.*mm"/>
-      <segmentation name="WSi_Pixel3" type="CartesianGridXY" key_value="23" grid_size_x="3.*mm" grid_size_y="3.*mm"/>
-      <segmentation name="WSi_Pad1"   type="CartesianGridXY" key_min="2" key_max="11" grid_size_x="1.*cm" grid_size_y="1.*cm"/>
-      <segmentation name="WSi_Pad2"   type="CartesianGridXY" key_min="13" key_max="22" grid_size_x="1.*cm" grid_size_y="1.*cm"/>
+      <segmentation type="MultiSegmentation" key="silicon">
+        <segmentation name="WSi_Pixel1" type="CartesianGridXY" key_value="1" grid_size_x="3.*mm" grid_size_y="3.*mm"/>
+        <segmentation name="WSi_Pixel2" type="CartesianGridXY" key_value="12" grid_size_x="3.*mm" grid_size_y="3.*mm"/>
+        <segmentation name="WSi_Pixel3" type="CartesianGridXY" key_value="23" grid_size_x="3.*mm" grid_size_y="3.*mm"/>
+        <segmentation name="WSi_Pad1"   type="CartesianGridXY" key_min="2" key_max="11" grid_size_x="1.*cm" grid_size_y="1.*cm"/>
+        <segmentation name="WSi_Pad2"   type="CartesianGridXY" key_min="13" key_max="22" grid_size_x="1.*cm" grid_size_y="1.*cm"/>
+      </segmentation>
       <id>system:8,silicon:6,x:24:-12,y:-12</id>
     </readout>
   </readouts>