Fix new unused_mut detected by nightly-2023-04-30

    warning: variable does not need to be mutable
       --> src/ser.rs:464:22
        |
    464 |     fn serialize_map(mut self, len: Option<usize>) -> Result<Self::SerializeMap> {
        |                      ----^^^^
        |                      |
        |                      help: remove this `mut`
        |
        = note: `#[warn(unused_mut)]` on by default

src/ser.rs

@@ -461,7 +461,7 @@ where
         Ok(self)
     }
 
-    fn serialize_map(mut self, len: Option<usize>) -> Result<Self::SerializeMap> {
+    fn serialize_map(self, len: Option<usize>) -> Result<Self::SerializeMap> {
         if len == Some(1) {
             self.state = if let State::FoundTag(_) = self.state {
                 self.emit_mapping_start()?;