fix sectioning: cannot skip section levels

Doc/lib/libast.tex

@@ -47,7 +47,7 @@ question mark), the value might be \code{None}. If the attributes
 can have zero-or-more values (marked with an asterisk), the
 values are represented as Python lists.
 
-\subsection{Abstract Grammar}
+\section{Abstract Grammar}
 
 The module defines a string constant \code{__version__} which
 is the decimal subversion revision number of the file shown below.