expo: Fix documentation reference

Fix typo: doc/develop/expo.rst instead of doc/developer/expo.rst

Signed-off-by: Massimo Pegorer <massimo.pegorer+oss@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
2 files changed