The instructions that appear on the Moodle login page are stored in the Moodle configuration property auth_instructions. You can edit them from Site Administration > Plugins > Authentication > Manage Authentication (instructions).
In the database, look for auth_instructions in the
mdl_config table (e.g
select * from mdl_config where name = 'auth_instructions') and change the value (which can be HTML).
You may need to purge your Moodle cache to see changes.