Simplify 'Enumeration Value' -> 'Enumeration' in headings

The shorter heading increases the visibility of the final "Integer", and I don't see any real risk of misinterpretation here.

An alternative improvement of this heading would have been to just change 'Values' -> 'Value'.

chapters/classes.tex

@@ -1609,7 +1609,7 @@ \subsubsection{Attributes of Enumeration Types}\label{attributes-of-enumeration-
 (\lstinline!quantity!, \lstinline!min!, \lstinline!max!, \lstinline!start!, \lstinline!fixed!) as enumeration literals.
 \end{nonnormative}
 
-\subsubsection{Enumeration Values Conversion to String or Integer}\label{enumeration-values-conversion-to-string-or-integer}\label{type-conversion-of-enumeration-values-to-string-or-integer}
+\subsubsection{Conversion of Enumeration to String or Integer}\label{conversion-of-enumeration-to-string-or-integer}\label{type-conversion-of-enumeration-values-to-string-or-integer}
 
 % TODO: Can't have angle brackets and \emph in the same mathescape due to LaTeXML issue:
 % - https://github.com/brucemiller/LaTeXML/issues/1477
@@ -1624,7 +1624,7 @@ \subsubsection{Enumeration Values Conversion to String or Integer}\label{enumera
 
 See also \cref{numeric-functions-and-conversion-functions}.
 
-\subsubsection{Integer Conversion to Enumeration Value}\label{integer-conversion-to-enumeration-value}\label{type-conversion-of-integer-to-enumeration-values}
+\subsubsection{Conversion of Integer to Enumeration}\label{conversion-of-integer-to-enumeration}\label{type-conversion-of-integer-to-enumeration-values}
 
 Whenever an enumeration type is defined, a type conversion function with
 the same name and in the same scope as the enumeration type is