Fix inconsistent variable names in the ListView sample code (#1902)

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## Description
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This PR will fix inconsistency in the sample code for `ListView`s with
filtering.

## Motivation and Context
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## How Has This Been Tested?
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Ran the sample app and confirmed the changes.

## Screenshots (if appropriate):

## Types of changes
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the boxes that apply: -->
- [x] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)

Author: AndrewKeepCoding

WinUIGallery/Samples/SampleCode/ListView/ListViewSample4_cs.txt

@@ -44,13 +44,13 @@ from the original allContacts list that are now supposed to be displayed (i.e. w
 
 private void Remove_NonMatching(IEnumerable<Contact> filteredData)
 {
-    for (int i = contacts3Filtered.Count - 1; i >= 0; i--)
+    for (int i = contactsFiltered.Count - 1; i >= 0; i--)
     {
-        var item = contacts3Filtered[i];
+        var item = contactsFiltered[i];
         // If contact is not in the filtered argument list, remove it from the ListView's source.
         if (!filteredData.Contains(item))
         {
-            contacts3Filtered.Remove(item);
+            contactsFiltered.Remove(item);
         }
     }
 }
@@ -60,9 +60,9 @@ private void AddBack_Contacts(IEnumerable<Contact> filteredData)
     foreach (var item in filteredData)
     {
         // If item in filtered list is not currently in ListView's source collection, add it back in
-        if (!contacts3Filtered.Contains(item))
+        if (!contactsFiltered.Contains(item))
         {
-            contacts3Filtered.Add(item);
+            contactsFiltered.Add(item);
         }
     }
 }