WARNING: Title underline too short.

[andylolz]

A bunch of titles have underlines that are too short, and cause build warnings:

/IATI-Standard-SSOT/docs/en/guidance/decimal-upgrade-to-2-03/2-03-changes.rst:68: WARNING: Title underline too short.
/IATI-Standard-SSOT/docs/en/guidance/decimal-upgrade-to-2-03/2-03-changes.rst:68: WARNING: Title underline too short.
/IATI-Standard-SSOT/docs/en/guidance/decimal-upgrade-to-2-03/2-03-changes.rst:73: WARNING: Title underline too short.
/IATI-Standard-SSOT/docs/en/guidance/decimal-upgrade-to-2-03/2-03-changes.rst:73: WARNING: Title underline too short.
/IATI-Standard-SSOT/docs/en/guidance/decimal-upgrade-to-2-03/2-03-changes.rst:95: WARNING: Title underline too short.
/IATI-Standard-SSOT/docs/en/guidance/implementation-schedule.rst:25: WARNING: Title underline too short.
/IATI-Standard-SSOT/docs/en/guidance/implementation-schedule.rst:25: WARNING: Title underline too short.
/IATI-Standard-SSOT/docs/en/guidance/reference/structure-of-data.rst:2: WARNING: Title underline too short.
/IATI-Standard-SSOT/docs/en/guidance/select-publishing-tool.rst:2: WARNING: Title underline too short.
/IATI-Standard-SSOT/docs/en/guidance/select-publishing-tool.rst:7: WARNING: Title underline too short.
/IATI-Standard-SSOT/docs/en/guidance/select-publishing-tool.rst:44: WARNING: Title underline too short.
/IATI-Standard-SSOT/docs/en/guidance/select-publishing-tool.rst:44: WARNING: Title underline too short.
/IATI-Standard-SSOT/docs/en/guidance/select-publishing-tool.rst:57: WARNING: Title underline too short.
/IATI-Standard-SSOT/docs/en/guidance/select-publishing-tool.rst:57: WARNING: Title underline too short.
/IATI-Standard-SSOT/docs/en/guidance/select-publishing-tool.rst:64: WARNING: Title underline too short.
/IATI-Standard-SSOT/docs/en/guidance/select-publishing-tool.rst:64: WARNING: Title underline too short.