Fix typo in GetTagItem() summary (#9812)

Author: andreycha

xml/System.Diagnostics/Activity.xml

@@ -862,7 +862,7 @@ Normally if the <xref:System.Diagnostics.Activity.ParentId> is defined, its form
       </Parameters>
       <Docs>
         <param name="key">The tag key string.</param>
-        <summary>Returns the value of the Activity tag mapped to the input key/&gt;.
+        <summary>Returns the value of the Activity tag mapped to the input key.
             Returns <see langword="null" /> if that key does not exist.</summary>
         <returns>The tag value mapped to the input key.</returns>
         <remarks>To be added.</remarks>