remove _ from label: LaTeX rejects this due to all the magic we use to

get the _ character to format like a normal character

Doc/ref/ref3.tex

@@ -1249,7 +1249,7 @@ owner class.
 \end{methoddesc}
 
 
-\subsubsection{Invoking Descriptors \label{descriptor_invocation}}
+\subsubsection{Invoking Descriptors \label{descriptor-invocation}}
 
 In general, a descriptor is an object attribute with ``binding behavior'',
 one whose attribute access has been overridden by methods in the descriptor