Fix some inaccuracies in haskell.html.markdown

- The bottom of the "List and Tuples" section may mislead the reader
  into thinking that the `fst` and `snd` functions can be applied to any
  tuple; it's worth mentioning that those functions only apply to pairs.

- The example demonstrating the use of the function-application operator
  (`$`) in combination with the function-composition operator (`.`) seems a
  bit contrived. For completeness, I've added an example that uses `$` alone.

- "If statements" and "case statements" are actually expressions, in
  Haskell; I've replaced all occurences of the word "statement" appearing in
  that context by the word "expression".

- Minor wording improvement (replaced "because" by a semicolon).