Update GC.xml (dotnet#9448)

Author: kevingosse

xml/System/GC.xml

@@ -1660,7 +1660,7 @@ The following example demonstrates the use of the <xref:System.GC.GetGeneration%
       </Parameters>
       <Docs>
         <param name="totalSize">The total size of the no GC region.</param>
-        <param name="callback">The callback to be executed a certain amount of memory is allocated in the no GC region.</param>
+        <param name="callback">The callback to be executed when a certain amount of memory is allocated in the no GC region.</param>
         <summary>Registers a callback to be invoked when a certain amount of memory is allocated in the no GC region.</summary>
         <remarks>To be added.</remarks>
         <exception cref="T:System.ArgumentOutOfRangeException">