support/testing/infra/emulator.py: fix typo in comment

Artefact (British) and Artifact (American) are both valid spelling
but ARTIFACTS_URL is used in the emulator code.

Surprisingly, the url actually use "artefacts"

  http://autobuild.buildroot.net/artefacts

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
(cherry picked from commit e0e64e9edeafd6467d20d211463ad28b20705503)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

support/testing/infra/emulator.py

@@ -20,7 +20,7 @@ class Emulator(object):
     #
     # kernel: path to the kernel image, or the special string
     # 'builtin'. 'builtin' means a pre-built kernel image will be
-    # downloaded from ARTEFACTS_URL and suitable options are
+    # downloaded from ARTIFACTS_URL and suitable options are
     # automatically passed to qemu and added to the kernel cmdline. So
     # far only armv5, armv7 and i386 builtin kernels are available.
     # If None, then no kernel is used, and we assume a bootable device