Fix summary on Diagnostics.Activity.RootId (#10281)

Fixes #10205

Author: rzikm

xml/System.Diagnostics/Activity.xml

@@ -1299,7 +1299,7 @@ If the <xref:System.Diagnostics.Activity.ParentId?displayProperty=nameWithType>
       </ReturnValue>
       <Docs>
         <summary>Gets the root ID of this <see cref="T:System.Diagnostics.Activity" />.</summary>
-        <value>The root ID, or <see langword="null" /> if the current instance has either a <see cref="P:System.Diagnostics.Activity.ParentId" /> or an <see cref="P:System.Diagnostics.Activity.Id" />.</value>
+        <value>The root ID, or <see langword="null" /> if the current instance has neither <see cref="P:System.Diagnostics.Activity.ParentId" /> nor <see cref="P:System.Diagnostics.Activity.Id" />.</value>
         <remarks>
           <format type="text/markdown"><![CDATA[