fix a versionchanged tag

Doc/lib/libcfgparser.tex

@@ -61,7 +61,7 @@ is given, it will be used to create the dictionary objects for the list
 of sections, for the options within a section, and for the default values.
 This class does not support the magical interpolation behavior.
 \versionadded{2.3}
-\versionchanged{\var{dict_type} was added}[2.6]
+\versionchanged[\var{dict_type} was added]{2.6}
 \end{classdesc}
 
 \begin{classdesc}{ConfigParser}{\optional{defaults}}