minor clarification for matching pairs (documentation)

In retrospect it is pretty obvious, but I needed to look up the associativity of `=>` to figure out how to match `Pair(x,y)` expressions, ie
```julia
@match :(a => b) begin
(x_ => y_) => (x,y) # need parentheses
_ => :unmatched
end
```
where `x_ => y_ => (x,y)` won't work because it is parsed as `x_ => (y_ => (x,y))`. Thanks for the great package, if you consider this issue too trivial just close it.