I have solved the problem. It was the existence of an empty line between the line : "objectClass: olcSchemaConfig" and the line "olcObjectClasses: ( 1.3.6.1.4.1.42.........". Once i deleted the empty line everything worked well.

#Definición de ObjectClass "stPerson" y atributo "correo"
dn: cn=stSchema,cn=schema,cn=config
objectClass: olcSchemaConfig
olcObjectClasses: ( 1.3.6.1.4.1.4203.666.1.1 NAME 'stPerson' SUP inetOrgPerson STRUCTURAL MUST correo)
olcAttributeTypes: ( 1.3.6.1.4.1.4203.666.1.2 NAME 'correo' SUP mail)