Manual:$wgAllowAnonymousMinor

User rights, access control and monitoring: $wgAllowAnonymousMinor
Allow anonymous users to mark changes as "minor"
Introduced in version:1.2.0
Removed in version:1.6.0
Allowed values:(boolean)
Default value:false

Details edit

Allow anonymous users to mark changes as "minor"

Another way to do this is adding this to LocalSettings :

$wgGroupPermissions['*']['minoredit']    = false;