Update 03-types.md

Using U_i instead of T_i is confusing. See http://stackoverflow.com/questions/26803244/is-this-a-typo-in-the-scala-language-specification-on-parameterized-types

Author: jhegedus42

spec/03-types.md

@@ -167,8 +167,8 @@ SimpleType      ::=  SimpleType TypeArgs
 TypeArgs        ::=  ‘[’ Types ‘]’
 ```
 
-A parameterized type $T[ U_1 , \ldots , U_n ]$ consists of a type
-designator $T$ and type parameters $U_1 , \ldots , U_n$ where
+A parameterized type $T[ T_1 , \ldots , T_n ]$ consists of a type
+designator $T$ and type parameters $T_1 , \ldots , T_n$ where
 $n \geq 1$. $T$ must refer to a type constructor which takes $n$ type
 parameters $a_1 , \ldots , a_n$.