Update readonly.md

Author: basarat

docs/types/readonly.md

@@ -52,7 +52,7 @@ type Foo = {
 
 type FooReadonly = Readonly<Foo>; 
 
-let foo: FOO = {bar: 123, bas: 456};
+let foo: Foo = {bar: 123, bas: 456};
 let fooReadonly: FooReadonly = {bar: 123, bas: 456};
 
 foo.bar = 456; // Okay