Fix a typo in a comment.

compiler/rustc_ast/src/tokenstream.rs

@@ -224,7 +224,7 @@ pub fn to_tokenstream(&self) -> TokenStream {
                                 // Inner attributes are only supported on extern blocks, functions,
                                 // impls, and modules. All of these have their inner attributes
                                 // placed at the beginning of the rightmost outermost braced group:
-                                // e.g. fn foo() { #![my_attr} }
+                                // e.g. fn foo() { #![my_attr] }
                                 //
                                 // Therefore, we can insert them back into the right location
                                 // without needing to do any extra position tracking.