Update comment re: private method rename: _add_ctes
### Describe the bug
In #2834, it looks like we switched the name of the internal method from `_insert_ctes` to `_add_ctes`. That's fine, we should just also update the comment that serves as a hint for plugin authors:

https://github.com/fishtown-analytics/dbt/blob/3af02020ff697cbc34b7bf7d538e99ccd1979afd/core/dbt/compilation.py#L353-L358

### Context
Adapter plugin authors for databases that don't support nested CTEs should have a clear point of intervention to alter dbt's behavior when constructing data tests and ephemeral models:
- [dbt-synapse#25](https://github.com/dbt-msft/dbt-synapse/issues/25)
- [dbt-sqlserver#26](https://github.com/mikaelene/dbt-sqlserver/issues/26)

If we have a little extra time, I'd like to prove it's actually possible to override this behavior for data tests and ephemeral models by overriding `_add_ctes` in an adapter plugin.
diff --git a/core/dbt/compilation.py b/core/dbt/compilation.py
index f66b6806a..0d6a4ccb9 100644
--- a/core/dbt/compilation.py
+++ b/core/dbt/compilation.py
@@ -348,13 +348,13 @@ class Compiler:
         manifest: Manifest,
         extra_context: Dict[str, Any],
     ) -> NonSourceCompiledNode:
-        """Insert the CTEs for the model."""
+        """Wrap the data test SQL in a CTE."""
 
         # for data tests, we need to insert a special CTE at the end of the
         # list containing the test query, and then have the "real" query be a
         # select count(*) from that model.
-        # the benefit of doing it this way is that _insert_ctes() can be
-        # rewritten for different adapters to handle databses that don't
+        # the benefit of doing it this way is that _add_ctes() can be
+        # rewritten for different adapters to handle databases that don't
         # support CTEs, or at least don't have full support.
         if isinstance(compiled_node, CompiledDataTestNode):
             # the last prepend (so last in order) should be the data test body.
['core/dbt/compilation.py']