Move 'Flattening' section in solvers/README.md

Author: John Nonweiler

src/solvers/README.md

@@ -60,12 +60,12 @@ different decision procedures, roughly one per directory.
 
 * sat/:   Back-ends for a variety of SAT solvers and DIMACS output.
 
-\section solver-apis Solver APIs
-
-\subsection flattening-section Flattening
+\section flattening-section Flattening
 
 To be documented.
 
+\section solver-apis Solver APIs
+
 \subsection smt-solving-api-section SMT solving API
 
 To be documented.