[spec] Fix spec for execution of struct.new, array.new_fixed and br_on_cast(_fail)

[f52985]

Hello,
This pull request fixes some minor errors in the specification for execution of struct.new, array.new_fixed and br_on_cast(_fail) instructions.

1. Change the last Return ... in prose notation specification into Push ... to the stack for both new instructions.
2. Change array ft^n in formal notation specification into array ft for array.new_fixed instruction.
3. Add the frame F in the reduction rule of br_on_cast(_fail), as F is used in the auxiliary function clos.
4. Fix typo: X{rt}_2 ~> \X{rt}_2for br_on_cast(_fail) instructions.

[rossberg]

Great, thanks!