Fix xdot command example in cpp tutorial
[Documentation fix]
In recent versions of xdot, piping the output of `bazel query` no
longer displays the visualization due to an unrelated change in xdot.
So, use redirection instead of pipe.
Closes
@@ -203,8 +203,8 @@ Then you can generate and view the graph by piping the text output above
straight to xdot:
```
-bazel query --nohost_deps --noimplicit_deps 'deps(//main:hello-world)' \
- --output graph | xdot
+xdot <(bazel query --nohost_deps --noimplicit_deps 'deps(//main:hello-world)' \
+ --output graph)
```
As you can see, the first stage of the sample project has a single target
