HolSmt: parse assign-bounds index in arith rules

This commit fixes a parsing error when parsing arithmetic `th-lemma`
inference rules with an `assign-bounds` parameter, i.e.:

((_ th-lemma arith assign-bounds ...) ...)

The error was due to `assign-bounds` not being in the known term
dictionary.

src/HolSmt/Z3_ProofParser.sml

@@ -111,6 +111,8 @@ local
     ("and-elim",        one_prem "and-elim"),
     (* the following is used in `(_ th-lemma arith ...)` inference rules *)
     ("arith",           builtin_name "arith"),
+    (* the following is used in `(_ th-lemma arith ...)` inference rules *)
+    ("assign-bounds",   builtin_name "assign-bounds"),
     (* the following is used in `(_ th-lemma bv ...)` inference rules *)
     ("bv",              builtin_name "bv"),
     ("asserted",        zero_prems "asserted"),