book: Some operator fixes for the syntax index
book: Some operator fixes for the syntax index

- Correct the names of the comparison traits (PartialOrd)
- Mention only the traits that overload the operator (PartialOrd,
  PartialEq), not operator-unrelated traits (Ord, Eq).
- Add `!=` operator.

GBGamer/rust,aidancully/rust,GBGamer/rust,graydon/rust,GBGamer/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,GBGamer/rust,GBGamer/rust