seastar: pick up document fixes

pick up changes to address the split script failure with HTML generated
with the latest pandoc.

Signed-off-by: Kefu Chai <kchai@redhat.com>

diff --git a/src/seastar b/src/seastar
index 2b392dec33e91c1fc5fb9eea04fbc08b078a874a..8c8fd3ed283329a521f363fe5fbbdf238efc415e 160000 (submodule)
--- a/src/seastar
+++ b/src/seastar
@@ -1 +1 @@
-Subproject commit 2b392dec33e91c1fc5fb9eea04fbc08b078a874a
+Subproject commit 8c8fd3ed283329a521f363fe5fbbdf238efc415e