blog: fix date formatting for express announcement

[Fishrock123]

refs: #531 & #530

r= @mikeal

[ghost]

good catch on the spaces there.

[Fishrock123]

@hg oh uh, that wasn't really intentional, my editor does it automatically (Atom). If we don't want that for markdown we should add an .editorconfig like core.

[stevemao]

@Fishrock123 I really hate using trailing spaces for newlines. We should avoid it.