doc/sphinx/requirements.txt: Bump certifi up

Upgrade certifi to the latest version, to remove e-Tugra from the root
store.

Link: https://groups.google.com/a/mozilla.org/g/dev-security-policy/c/C-HrP1SEq1A?pli=1
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
diff --git a/doc/sphinx/requirements.txt b/doc/sphinx/requirements.txt
index b74661a..4f411f7 100644
--- a/doc/sphinx/requirements.txt
+++ b/doc/sphinx/requirements.txt
@@ -1,6 +1,6 @@
 alabaster==0.7.12
 Babel==2.9.1
-certifi==2023.5.7
+certifi==2023.07.22
 charset-normalizer==2.0.12
 docutils==0.16
 idna==3.3