Fixed use of mk_th_axiom in theory_fpa.
Fixed use of mk_th_axiom in theory_fpa.

Relates to #227

seahorn/z3,conceptofproof/z3,seahorn/z3,conceptofproof/z3,mschlaipfer/z3,conceptofproof/z3,kyledewey/z3,conceptofproof/z3,mschlaipfer/z3,kyledewey/z3,seahorn/z3,conceptofproof/z3,kyledewey/z3,kyledewey/z3,conceptofproof/z3,kyledewey/z3,wujunzero/z3,wujunzero/z3,seahorn/z3,mschlaipfer/z3,mschlaipfer/z3,wujunzero/z3,wujunzero/z3,kyledewey/z3,wujunzero/z3,mschlaipfer/z3,seahorn/z3,seahorn/z3,mschlaipfer/z3,wujunzero/z3