Fix `logic.rs` comments

[davidnevadoc]

The logic gates are explained with formulas drawn in the comments. Automatic formatting has shifted this drawings making some of them incomprehensible. Example:

  //                    /                 \          /
  // \  c      - 4 . c  = | a      - 4 . a  | (& OR ^) | b
  // - 4 . b  |   i + 1        i   \  i + 1        i /
  // \  i + 1        i /
  //

Originally:

//                    /                 \          /                 \
//  c      - 4 . c  = | a      - 4 . a  | (& OR ^) | b      - 4 . b  |
//   i + 1        i   \  i + 1        i /          \  i + 1        i /
//

[markulf]

This is still open.

Found these comments in plonk-core/src/constraint_system/logic.rs