[DOC] using Math::N in examples

Author: nobu

README.md

@@ -13,6 +13,8 @@ Without mathn:
 With mathn:
 
 ```ruby
+require "mathn"
+using Math::N
 3 / 2 => 3/2 # Rational
 ```
 
@@ -27,14 +29,16 @@ Without mathn:
 With mathn:
 
 ```ruby
+require "mathn"
+using Math::N
 20 / 9 * 3 * 14 / 7 * 3 / 2 # => 20
 ```
 
 ## Global Behavioral changes
 
 While older version of 'mathn', just by required, caused changes to
 the behavior (and even the types) of operations on classes like
-Integer newer `mathn` introduces the refinements `Math::N`, and you
+Integer, newer `mathn` introduces the refinements `Math::N`, and you
 have to enable it.
 
 Before ruby 2.5, `mathn` was part of the ruby standard library. It was