Fix indentation of parenthesized Expressions#to_s

[HertzDevil]

Before:

if true
(  1)
else
  (  1)
  (  @x = x).is_a?(A)
  2
end

After:

if true
  (1)
else
  (1)
  (@x = x).is_a?(A)
  2
end

[straight-shoota]

This isn't just macro interpolation but Expressions#to_s.