Clarify fn keyword's usage as argument

[KorvinSzanto]

This PR removes the "preceding space" requirement from the fn keyword used in short closures in order to allow using short closures as arguments without needing to include a preceeding space.

// Current spec:
foo( fn(int $x, int $y): int => $x + $y);

// After this PR:
foo(fn(int $x, int $y): int => $x + $y);

[theodorejb]

Good catch. 👍