fix: make merged ContainerBlock docstring pydocstyle-compliant

ContainerBlock (#1949) landed on main before the D-rules were enabled and
was merged into this branch, tripping D205/D415. Add the blank line after
the summary so `ruff check` passes tree-wide.

Co-Authored-By: Claude <svc-devxp-claude@slack-corp.com>

Author: WilliamBergamin

slack_sdk/models/blocks/blocks.py

@@ -1064,6 +1064,7 @@ def __init__(
         **others: dict,
     ):
         """A general-purpose wrapper for grouping child blocks together, with a configurable size.
+
         https://docs.slack.dev/reference/block-kit/blocks/container-block
 
         Args: