Change the return type of Route::stage_instances to &'static str (#…

…1675)

Functions without parameters should return the static string directly.

BREAKING CHANGE: The function `Route::stage_instances` now returns
a `&'static str` instead of a `String`.

src/http/routing.rs

@@ -1031,8 +1031,8 @@ impl Route {
         api!("/applications/{}/guilds/{}/commands/permissions", application_id, guild_id)
     }
 
-    pub fn stage_instances() -> String {
-        api!("/stage-instances").to_string()
+    pub fn stage_instances() -> &'static str {
+        api!("/stage-instances")
     }
 
     pub fn stage_instance(channel_id: u64) -> String {