Minor typo in TEST_CASE("Addable")

[reneparis]

Hey everybody,

First of all: Gorgeous work!
Then: I guess the AddableType should use an AddableTag instead of a SubtractableTag ;)

NamedType/main.cpp

Line 101 in 27cb8d9

using AddableType = fluent::NamedType<int, struct SubtractableTag, fluent::Addable>;

[knatten]

Fixed in #32