use the async keyword as syntax in the macro invocation

crates/wiggle/tests/atoms_async.rs

@@ -7,7 +7,7 @@ use wiggle_test::{impl_errno, HostMemory, MemArea, WasiCtx};
 
 wiggle::from_witx!({
     witx: ["$CARGO_MANIFEST_DIR/tests/atoms.witx"],
-    async_: {
+    async: {
         atoms::{int_float_args, double_int_return_float}
     }
 });