Fix description of serializer public API convention

Author: dtolnay

src/ser.rs

@@ -15,7 +15,7 @@ pub struct Serializer {
     output: String,
 }
 
-// By convention, the public API of a Serde deserializer is one or more `to_abc`
+// By convention, the public API of a Serde serializer is one or more `to_abc`
 // functions such as `to_string`, `to_bytes`, or `to_writer` depending on what
 // Rust types the serializer is able to produce as output.
 //