Add a missing default value to docs (#3973)

Release Notes:

- N/A

crates/call/src/call_settings.rs

@@ -9,9 +9,12 @@ pub struct CallSettings {
     pub mute_on_join: bool,
 }
 
+/// Configuration of voice calls in Zed.
 #[derive(Clone, Default, Serialize, Deserialize, JsonSchema, Debug)]
 pub struct CallSettingsContent {
     /// Whether the microphone should be muted when joining a channel or a call.
+    ///
+    /// Default: false
     pub mute_on_join: Option<bool>,
 }