Fix comment for async closure variant

Author: liuw

compiler/rustc_hir/src/hir.rs

@@ -1441,7 +1441,7 @@ pub enum AsyncGeneratorKind {
     /// An explicit `async` block written by the user.
     Block,
 
-    /// An explicit `async` block written by the user.
+    /// An explicit `async` closure written by the user.
     Closure,
 
     /// The `async` block generated as the body of an async function.