message: Fix new custom types syntax error in the document
diff: -# Version 2.0 Syntax (2019-11-11)
  +# Version 2.0 Syntax (2019-12-03)
  The ink! version 2.0 syntax has one major philosophy:
  @@ -92,7 +92,7 @@ We used to define types using a special `#![env = DefaultSrmlTypes]` tag.
  Now we simply include the type definition in the `#[ink::contract(...)]` tag:
  ```rust
  -#[ink::contract(version = "0.1.0", types = MyCustomTypes)]
  +#[ink::contract(version = "0.1.0", env = MyCustomTypes)]
  ```
  By default, we use `DefaultSrmlTypes`, so you don't need to define anything unless you plan to use
  