Fix incorrect curly quotes

Author: steveklabnik

src/doc/trpl/traits.md

@@ -192,7 +192,7 @@ Here’s the error:
 ```text
 error: type `std::fs::File` does not implement any method in scope named `write`
 
-let result = f.write(b”whatever”);
+let result = f.write(b"whatever");
                ^~~~~~~~~~~~~~~~~~
 ```