[formatter] async.nim uses two spaces (#19264)

according to https://nim-lang.org/docs/nep1.html#introduction-spacing-and-whitespace-conventions, two spaces should be preferred in stdlib.

lib/pure/async.nim

@@ -1,6 +1,6 @@
 when defined(js):
-    import asyncjs
-    export asyncjs
+  import asyncjs
+  export asyncjs
 else:
-    import asyncmacro, asyncfutures
-    export asyncmacro, asyncfutures
+  import asyncmacro, asyncfutures
+  export asyncmacro, asyncfutures