Fix OBO readme link

Author: omaus

README.md

@@ -10,7 +10,7 @@ Small usage instructions, examples and code snippets can be found under the READ
 
 - [ControlledVocabulary]()
 - [Ontology](https://github.com/CSBiology/Ontology.NET/tree/main/src/Ontology.NET#readme)
-- [OBO]()
+- [OBO](https://github.com/CSBiology/Ontology.NET/tree/main/src/Ontology.NET/OBO#readme)
 - [Ontology Extensions]()
 
 ## Contributing
@@ -97,3 +97,4 @@ where `<arguments>` can be:
 - `semver:<version>` where <version> can be `major`, `minor` or `patch`. The respective version part is raised by +1.
 - `pre:<previewName>` where <previewName> is freetext.
 - `n:<n>` where <n> is the number of commits that (shall) belong to the new release. Default is 30.
+