fix: typo in uproot.source.xrootd.XRootDSource name

[jpivarski]

I scanned all of @lobis's PRs for "XRootDSource" and "source.root" and it only appeared in #971.

We need a test for it, though.

[lobis]

@jpivarski I added a test for it. uproot.open was not being tested against xrootd files (the source was being used directly) that is why this typo had been passing the tests. (Perhaps this test should be in another file though, I made a quick fix using pytest fixtures).

[jpivarski]

Most of the Sources tests were very early, around test_0001_*.py through test_0005_*.py. They were probably written before uproot.open existed, and that's how this slipped through. I know there are uproot.open uses with HTTP in other parts of the test suite (testing things other than merely being able to open the file with HTTP).

But this test is a positive improvement: it's walking through the control flow that leads from a URL with root:// to loading the XRootDSource, and would have caught the bug. So for now, at least, I'll merge this and make Uproot 5.1.1.

I'm not allowed to approve this PR, probably because I started it. Oh well, I'll merge without approval.