update context_url in json2jsonld.py

[proccaserra]

Detected by error message generated when loading jsonld serialisation of ISA using rdflib: HTTPError 404 Not Found

at line 205:

context_url = (
            "https://raw.githubusercontent.com/ISA-tools/isa-api/develop/isatools/"
            "resources/json-context/%s/isa_" % self.ontology
        ) 

url should be pointing to 'main/master' branch.

url should have the following pattern:

https://raw.githubusercontent.com/ISA-tools/isa-api/refs/heads/master/isatools/resources/json-context/wd/isa_<object>_<semantic domain>_context.jsonld