Clear up warning from new Rust version

postman/src/sender.rs

@@ -151,7 +151,7 @@ impl Embed {
                 embed.description = substitute(&embed.description);
                 embed.footer = substitute(&embed.footer);
 
-                embed.fields.iter_mut().for_each(|mut field| {
+                embed.fields.iter_mut().for_each(|field| {
                     field.title = substitute(&field.title);
                     field.value = substitute(&field.value);
                 });