EASY [PyText] fix doc sphinx deprecation warning (#775)

Summary:
Pull Request resolved: #775

Before the fix, `make html` prints this warning.

  /Users/egaudet/src/pytext.src/pytext/docs/source/conf.py:204: RemovedInSphinx30Warning: app.override_domain() is deprecated. Use app.add_domain() with override option instead.

Reviewed By: chenyangyu1988

Differential Revision: D16228043

fbshipit-source-id: 8a37cd8c331d51ace7b802541d6fd923a2d17245

pytext/docs/source/conf.py

@@ -201,5 +201,5 @@ def resolve_xref(self, env, fromdocname, builder, typ, target, node, contnode):
 
 
 def setup(app):
-    app.override_domain(PatchedPythonDomain)
+    app.add_domain(PatchedPythonDomain, override=True)
     app.connect("builder-inited", run_apidoc)