Fix an error in README.md

[singularitti]

Running

julia> @argcheck det(a) < 0 DomainError()
ERROR: MethodError: no method matching DomainError()
Closest candidates are:
  DomainError(::Any) at boot.jl:258
  DomainError(::Any, ::Any) at boot.jl:259
Stacktrace:
 [1] top-level scope at /Users/qz/.julia/packages/ArgCheck/xX4DA/src/checks.jl:165

will throw a MethodError, because DomainError cannot have empty field. It should be either

julia> @argcheck det(a) < 0 DomainError
ERROR: DomainError with det(a) < 0 must hold. Got
det(a) => 0.07470301636255529:

or DomainError with some msg:

julia> @argcheck det(a) < 0 DomainError("error")
ERROR: DomainError with error:

Stacktrace:
 [1] top-level scope at /Users/qz/.julia/packages/ArgCheck/xX4DA/src/checks.jl:165

[jw3126]

Thanks!