message: add missing '`' in README.md
diff: A collection of numeric types and traits for Rust.
  This includes new types for big integers, rationals, and complex numbers,
  -new traits for generic programming on numeric properties like `Integer,
  +new traits for generic programming on numeric properties like `Integer`,
  and generic range iterators.
  [Documentation](http://rust-num.github.io/num)
  