Change doxygen subsection label

Doxygen was giving a warning message about "multiple use of section
label 'deprecated'", so this commit changes the section label.  There
are no references to this section, so the label is only changed in
one place.  (I can't find any other uses of this section label, so
the warning message seems to be wrong, but it doesn't hurt to fix it.)

Author: John Nonweiler

src/solvers/README.md

@@ -308,7 +308,7 @@ allocates a new string before calling a primitive.
     \copybrief add_axioms_for_parse_int
     \link add_axioms_for_parse_int More... \endlink
 
-\subsection deprecated Deprecated primitives:
+\subsection solvers-deprecated Deprecated primitives:
 
   * `cprover_string_concat_code_point`, `cprover_string_code_point_at`,
     `cprover_string_code_point_before`, `cprover_string_code_point_count`: