promise.mdx: replace deprecated pipe

with the new version

Author: theqp

pages/docs/manual/latest/promise.mdx

@@ -42,18 +42,18 @@ Using the [pipe operator](pipe.md):
 let myPromise = Js.Promise.make((~resolve, ~reject) => resolve(. 2))
 
 myPromise
-|> Js.Promise.then_(value => {
+-> Js.Promise.then_(value => {
      Js.log(value)
      Js.Promise.resolve(value + 2)
-   })
-|> Js.Promise.then_(value => {
+   }, _)
+-> Js.Promise.then_(value => {
      Js.log(value)
      Js.Promise.resolve(value + 3)
-   })
-|> Js.Promise.catch(err => {
+   }, _)
+-> Js.Promise.catch(err => {
      Js.log2("Failure!!", err)
      Js.Promise.resolve(-2)
-   })
+   }, _)
 ```
 ```js
 var myPromise = new Promise(function (resolve, reject) {