Fix a typo in the docs for Union constructors (#315)

Author: paulcadman

src/Dhall/Tutorial.hs

@@ -1115,7 +1115,7 @@ import Dhall
 -- common pattern:
 --
 -- >     let MyType = constructors < Empty : {} | Person : { name : Text, age : Natural } >
--- > in  [   MyType.Empty {}
+-- > in  [   MyType.Empty {=}
 -- >     ,   MyType.Person { name = "John", age = +23 }
 -- >     ,   MyType.Person { name = "Amy" , age = +25 }
 -- >     ]