docs(aio): update to remark syntax

aio/content/tutorial/toh-pt6.md

@@ -121,7 +121,7 @@ Add the file `in-memory-data.service.ts` in `app` with the following content:
 This file replaces `mock-heroes.ts`, which is now safe to delete.
 
 
-~~~ {.alert.is-helpful}
+<div class="alert is-helpful">
 
 
 
@@ -134,7 +134,7 @@ Read more about the in-memory web API in the
 section of the [HTTP Client](guide/server-communication#in-mem-web-api) page.
 
 
-~~~
+</div>
 
 
 
@@ -203,14 +203,14 @@ That's as easy as importing them from the RxJS library like this:
 
 
 
-~~~ {.l-sub-section}
+<div class="l-sub-section">
 
 
 
 You'll add more operators, and learn why you must do so, [later in this tutorial](tutorial/toh-pt6#rxjs-imports).
 
 
-~~~
+</div>
 
 
 
@@ -231,7 +231,7 @@ holds the array of heroes that the caller wants.
 So you grab that array and return it as the resolved Promise value.
 
 
-~~~ {.alert.is-important}
+<div class="alert is-important">
 
 
 
@@ -240,7 +240,7 @@ This particular in-memory web API example returns an object with a `data` proper
 Your API might return something else. Adjust the code to match your web API.
 
 
-~~~
+</div>
 
 
 
@@ -523,7 +523,7 @@ The `http.get()` call in `HeroSearchService` is similar to the one
 in the `HeroService`, although the URL now has a query string.
 
 More importantly, you no longer call `toPromise()`.
-Instead you return the *Observable* from the the `htttp.get()`,
+Instead you return the *Observable* from the the `http.get()`,
 after chaining it to another RxJS operator, <code>map()</code>,
 to extract heroes from the response data.
 RxJS operator chaining makes response processing easy and readable.
@@ -616,7 +616,7 @@ before passing along the latest string. You'll never make requests more frequent
 It cancels and discards previous search observables, returning only the latest search service observable.
 
 
-~~~ {.l-sub-section}
+<div class="l-sub-section">
 
 
 
@@ -637,7 +637,7 @@ Note that until the service supports that feature, _canceling_ the `HeroSearchSe
 doesn't actually abort a pending HTTP request.
 For now, unwanted results are discarded.
 
-~~~
+</div>
 
 
 
@@ -915,7 +915,7 @@ Here are the files you added or changed in this page.
 
 
 
-~~~ {.l-sub-section}
+<div class="l-sub-section">
 
 
 
@@ -924,5 +924,5 @@ Here are the files you added or changed in this page.
 Return to the [learning path](guide/learning-angular#architecture), where
 you can read more about the concepts and practices found in this tutorial.
 
-~~~
+</div>