message: Update reference to .stream() in ARCHITECTURE.md
diff: @@ -86,20 +86,14 @@ macros of the form `[MACRO]` are used in the expected output. For
  example, `[..]` matches any string.
  To see stdout and stderr streams of the subordinate process, add `.stream()`
  -call to `execs()`:
  +call to the built-up `Execs`:
  ```rust
  // Before
  -assert_that(
  - p.cargo("run"),
  - execs()
  -);
  +p.cargo("run").run();
  // After
  -assert_that(
  - p.cargo("run"),
  - execs().stream()
  -);
  +p.cargo("run").stream().run();
  ```
  Alternatively to build and run a custom version of cargo simply run `cargo build`
  