Fix typo of a type signature in simd_execute.v

[yoshihiro503]

Both vtestop and vshiftop are of type N, so the code compiles. However, shouldn't the type of the first argument to app_vshiftop_str be vshiftop rather than vtestop?

see also:

WasmCert-Coq/theories/operations.v

Lines 688 to 691 in 2e111b2

	Definition app_vshiftop (op: vshiftop) (v1: value_vec) (v2: i32) : value_vec :=
	let v1e := encode_vec v1 in
	let v2e := encode_i32 v2 in
	decode_vec (app_vshiftop_str op v1e v2e).

[raoxiaojia]

Indeed, this was probably a typo I made when I was copying the lines around. Thanks for the correction!