erp5_core: fix `TypeError` on raise in `Base_edit`

String value raised with `ValueError` had an extra `%s` at the end, hence
raising another error when formatting:

> TypeError: not enough arguments for format string

This commit simply removes the extra formatting character, since I could not
guess what should have been printed here.

product/ERP5/bootstrap/erp5_core/SkinTemplateItem/portal_skins/erp5_core/Base_edit.py

@@ -186,7 +186,7 @@ def editMatrixBox(matrixbox_field, matrixbox):
           else:
             raise ValueError("Could not create cell %s" % str(cell_index_tuple))
         else:
-          raise ValueError("Cell %s does not exist %s" % str(cell_index_tuple))
+          raise ValueError("Cell %s does not exist" % str(cell_index_tuple))
 
 field_prefix_len = len(field_prefix)