Shift selectors left in the MPI example

While it makes sense to align them across examples, this one has
an extra condition, so it's probably better to shift it left, so people
have a better chance of noticing it.

Signed-off-by: Michał Górny <[email protected]>

Author: mgorny

docs/how-to/advanced/cross-compilation.md

@@ -227,7 +227,7 @@ With MPI, openmpi is required for the build platform as the compiler wrappers ar
 ```yaml
 requirements:
   build:
-    - {{ mpi }}                             # [build_platform != target_platform and mpi == "openmpi"]
+    - {{ mpi }}  # [build_platform != target_platform and mpi == "openmpi"]
   host:
     - {{ mpi }}
   run: