made tutorial outputs more presentable (#555)

Closes https://github.com/ooni/probe/issues/1831

internal/tutorial/measurex/chapter12/README.md

@@ -79,7 +79,7 @@ is closed when done by `MeasureURLAndFollowRedirections`, so we leave the loop.
 Let us perform a vanilla run first:
 
 ```bash
-go run -race ./internal/tutorial/measurex/chapter12
+go run -race ./internal/tutorial/measurex/chapter12 | jq
 ```
 
 Take a look at the JSON. You should see several redirects

internal/tutorial/measurex/chapter12/main.go

@@ -80,7 +80,7 @@ func main() {
 // Let us perform a vanilla run first:
 //
 // ```bash
-// go run -race ./internal/tutorial/measurex/chapter12
+// go run -race ./internal/tutorial/measurex/chapter12 | jq
 // ```
 //
 // Take a look at the JSON. You should see several redirects