Remove redundant item closing tag from dotpoint list (#11600)

Author: Issung

xml/System.Diagnostics/Activity.xml

@@ -1878,7 +1878,7 @@ Calling the `Start` method does the following:
 - Sets <xref:System.Diagnostics.Activity.Parent> to hold <xref:System.Diagnostics.Activity.Current>.
 - Sets <xref:System.Diagnostics.Activity.Current> to this <xref:System.Diagnostics.Activity>.
 - If <xref:System.Diagnostics.Activity.StartTimeUtc> was not set previously, sets it to <xref:System.DateTime.UtcNow?displayProperty=nameWithType>.
-- Generates a unique <xref:System.Diagnostics.Activity.Id> for this activity.</item>
+- Generates a unique <xref:System.Diagnostics.Activity.Id> for this activity.
 
 Use <xref:System.Diagnostics.DiagnosticSource.StartActivity%2A?displayProperty=nameWithType> to start the <xref:System.Diagnostics.Activity> and write the start event.