That Vow is defined in the example

Refs #561

Author: JJ

doc/Language/concurrency.pod6

@@ -202,7 +202,7 @@ If you are creating a promise that you intend to keep or break yourself
 then you probably don't want any code that might receive the promise to
 inadvertently (or otherwise) keep or break the promise before you do.
 For this purpose there is the L<method vow|/type/Promise#method_vow>, which
-returns a L<Vow> object which becomes the only mechanism by which the promise
+returns a Vow object which becomes the only mechanism by which the promise
 can be kept or broken.  If an attempt to keep or break the Promise is made
 directly then the exception L<X::Promise::Vowed> will be thrown, as long
 as the vow object is kept private, the status of the promise is safe: