fix(measurex): use same keys of the OONI data format (#572)

This change should simplify the pipeline's job.

Reference issue: https://github.com/ooni/probe/issues/1817.

I previously dismissed this possibility, but now it seems clear it
is simpler to have a very tabular data format internally and to
convert such a format to OONI's data format when serializing.

The OONI data format is what the pipeline expects, but processing
is easier with a more linear/tabular format.

internal/tutorial/measurex/chapter08/README.md

@@ -101,6 +101,15 @@ This is it. The rest of the program is exactly the same.
 	for _, epnt := range httpEndpoints {
 		m.Endpoints = append(m.Endpoints, mx.HTTPEndpointGetWithoutCookies(ctx, epnt))
 	}
+```
+
+(Note that here, like in the previous chapter, we are not converting
+to the OONI data format. Rather, we're just dumping the internally
+used data structures. Exercise: can you modify this program to emit
+a JSON compliant with the OONI data format by using the proper]
+conversion functions exported by `measurex`?)
+
+```Go
 	print(m)
 }