fix bug with core_compilers not working properly

Signed-off-by: Gregory Becker <becker33@llnl.gov>

Author: becker33

outputs/stacks/examples/8.spack.stack.yaml

@@ -38,3 +38,5 @@ spack:
         hierarchy:
         - mpi
         - lapack
+        core_specs:
+        - '%gcc@11'

outputs/stacks/examples/9.spack.stack.yaml

@@ -38,6 +38,8 @@ spack:
         hierarchy:
         - mpi
         - lapack
+        core_specs:
+        - '%gcc@11'
         hash_length: 0
         include:
         - gcc

tutorial_stacks.rst

@@ -396,6 +396,7 @@ The next step is to add some basic configuration to our ``spack.yaml`` to genera
    :emphasize-lines: 31-40
 
 In these few lines of additional configuration we told Spack to generate ``lmod`` module files in a subdirectory named ``modules``, using a hierarchy comprising both ``lapack`` and ``mpi``.
+We've also configured it to place all specs built with our system compiler into the ``Core`` designation in the lmod hierarchy.
 
 We can generate the module files and use them with the following commands: