Update docs/dcp2_system_design.rst

Co-authored-by: Alegria Aclan <aaclan@ebi.ac.uk>

docs/dcp2_system_design.rst

@@ -486,7 +486,7 @@ of the descriptor.
 
 The importer will skip any attempt to import the externally referenced files.
 Downstream consumers of these file descriptors must be resilient to a row with
-a ```NULL`` value in the TDR ``file_id`` column and fall back to using the
+a ``NULL`` value in the TDR ``file_id`` column and fall back to using the
 ``drs_uri`` property of the descriptor JSON in the ``content`` column of the
 same row.