Fix the default namespace declared in OBO header

[althonos]

Hi!

The OBO 1.4 format mandates that the OBO namespaces used in OBO files are identifiers, not free text, so you should change the default namespace declared in the header from:

default-namespace: Dicty Phenotypes

to

default-namespace: dicty_phenotype

I don't think you are using namespaces to separate the ontology into fragments, so that's shouldn't affect anything else, but this is all that's needed to make ddpheno compliant with the OBO 1.4 syntax.