Update docs to match code changes

Signed-off-by: David Gardner <[email protected]>

Author: dagardner-nv

docs/source/developer_guide/guides/3_simple_cpp_stage.md

@@ -383,7 +383,7 @@ As mentioned in the previous section, our `_build_single` method needs to be upd
 
 ```python
 def _build_single(self, builder: mrc.Builder, input_node: mrc.SegmentObject) -> mrc.SegmentObject:
-    if self._build_cpp_node() and isinstance(self._input_type, ControlMessage):
+    if self._build_cpp_node() and issubclass(self._input_type, ControlMessage):
         from ._lib import pass_thru_cpp
 
         node = pass_thru_cpp.PassThruStage(builder, self.unique_name)
@@ -438,7 +438,7 @@ class PassThruStage(PassThruTypeMixin, GpuAndCpuMixin, SinglePortStage):
         return message
 
     def _build_single(self, builder: mrc.Builder, input_node: mrc.SegmentObject) -> mrc.SegmentObject:
-        if self._build_cpp_node() and isinstance(self._input_type, ControlMessage):
+        if self._build_cpp_node() and issubclass(self._input_type, ControlMessage):
             from ._lib import pass_thru_cpp
 
             node = pass_thru_cpp.PassThruStage(builder, self.unique_name)