docs: change name of API section

from 'Borg Backup API documentation' to 'API Documentation'

It is pretty clear which software the API docs are for imho.

setup.py

@@ -184,8 +184,8 @@ class build_api(Command):
         print("auto-generating API documentation")
         with open("docs/api.rst", "w") as doc:
             doc.write("""
-Borg Backup API documentation
-=============================
+API Documentation
+=================
 """)
             for mod in glob('borg/*.py') + glob('borg/*.pyx'):
                 print("examining module %s" % mod)