scripts: partition_manager: Fix dummy variable reference

Replaces a dummy variable with one in the undef list

Signed-off-by: Jamie McCrae <[email protected]>

Author: nordicjm

scripts/partition_manager/partition_manager.rst

@@ -548,7 +548,7 @@ After the ``nordic,pm-ext-flash`` value is set, you can place partitions in the
    # Name of partition
    external_plz:
      region: external_flash
-     size: CONFIG_EXTERNAL_PLZ_SIZE
+     size: CONFIG_SOME_INT
 
 .. note::