Fix dot call in AST docs

[tkf]

It seems current documentation

julia/doc/src/devdocs/ast.md

Line 62 in 668341e

| `a.(b)` | `(. a b)` |

does not match with the implementation:

> (julia-parse "a.(b)")
(|.| a (tuple b))

julia> :(a.(b)).args
2-element Array{Any,1}:
 :a
 :((b,))

[c42f]

This probably dates back to when this syntax had the same meaning as matlab?

In any case, this change correctly reflects the current dot call lowering.