[glibmm] AsyncResult: Fix grammar error, <tt>ify enumerator

[Daniel Boles <dboles src gnome org>]

commit aedf65588f71779ade0c5e7e77b4f75fd67f4497
Author: Daniel Boles <dboles src gnome org>
Date:   Sun May 20 16:15:45 2018 +0100

    AsyncResult: Fix grammar error, <tt>ify enumerator
    
    This was not a grammar error before C++ification! But let’s fix it
    anyway, and give the enumeration name some nice formatting as well.

 gio/src/asyncresult.hg | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/gio/src/asyncresult.hg b/gio/src/asyncresult.hg
index 5b73a551..295a271f 100644
--- a/gio/src/asyncresult.hg
+++ b/gio/src/asyncresult.hg
@@ -92,7 +92,7 @@ using SlotAsyncReady = sigc::slot<void(Glib::RefPtr<AsyncResult>&)>;
  * The async function could also take an optional Glib::Cancellable object, allowing the calling function to 
cancel the asynchronous operation.
  *
  * The callback for an asynchronous operation is called only once, and is always called, even in the case of 
a cancelled operation.
- * On cancellation the result is a ERROR_CANCELLED error.
+ * On cancellation, the result is an <tt>ERROR_CANCELLED</tt> error.
  *
  * @newin{2,16}
  */