chore: correct URL to PyTest isolation guide

Closes #1648

brownie/test/managers/master.py

@@ -64,7 +64,7 @@ def pytest_sessionfinish(self, session):
  raise pytest.UsageError(
  "xdist workers failed to collect tests. Ensure all test cases are "
  "isolated with the module_isolation or fn_isolation fixtures.\n\n"
- "https://eth-brownie.readthedocs.io/en/stable/tests.html#isolating-tests"
+ "https://eth-brownie.readthedocs.io/en/stable/tests-pytest-intro.html#pytest-fixtures-isolation"
  )
  build_path = self.project._build_path