docs(impl-serializer): correct crate import

it was importing from the `error` crate, which wasn't intentional. this now specifies that we're talking about our current crate's error module.

_src/impl-serializer.md

@@ -23,7 +23,7 @@ is used.
 #
 use serde::{ser, Serialize};
 
-use error::{Error, Result};
+use crate::error::{Error, Result};
 
 pub struct Serializer {
     // This string starts empty and JSON is appended as values are serialized.