docs: Fixing header length in known_issues.

Signed-off-by: Tim 'mithro' Ansell <tansell@google.com>

docs/known_issues.rst

diff --git a/docs/known_issues.rst b/docs/known_issues.rst
index 4c8292a..d7d1204 100644
--- a/docs/known_issues.rst
+++ b/docs/known_issues.rst
@@ -85,7 +85,7 @@
 The SKY130 Oklahoma State University provided standard cells library will be integrated at a future date (see `Issue #10 <https://github.com/google/skywater-pdk/issues/10>`_).
 
 :lib:`sky130_fd_sp_flash` - SKY130 Flash Build Space
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 The flash build space is currently empty, only a placeholder is currently provided.