DOC: fix typo in the debugging signal names

[yut23]

PR Summary

The relevant code is here:

yt/yt/startup_tasks.py

Lines 45 to 55 in 13dc4fb

	# We use two signals, SIGUSR1 and SIGUSR2. In a non-threaded environment,
	# we set up handlers to process these by printing the current stack and to
	# raise a RuntimeError. The latter can be used, inside pdb, to catch an error
	# and then examine the current stack.
	try:
	signal.signal(signal.SIGUSR1, signal_print_traceback)
	mylog.debug("SIGUSR1 registered for traceback printing")
	signal.signal(signal.SIGUSR2, signal_ipython)
	mylog.debug("SIGUSR2 registered for IPython Insertion")
	except (ValueError, RuntimeError, AttributeError): # Not in main thread
	pass

[neutrinoceros]

Docs build failed but the causelooks unrelated

Sphinx parallel build error:
nbsphinx.NotebookError: NoSuchKernel in cookbook/custom_colorbar_tickmarks.ipynb:
No such kernel named python3

I've never seen this one before so let's start by checking if it's reproducible: @yt-fido test this please

[neutrinoceros]

Yep, definitely reproducible, but still unrelated. Let me merge this and open a dedicated ticket.