Fix typo

7 jobs for master in 4 minutes and 11 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #392367
doctest

00:02:08

passed #392365
html

00:01:32

passed #392368
latexpdf

00:02:00

failed #392369
allowed to fail
linkcheck

00:04:10

failed #392370
spelling

00:01:40

manual #392366
allowed to fail manual
stop review
passed #392364
validate-voevents

00:00:35

 
Name Stage Failure
failed
linkcheck Build
(line    7) ok        https://lscsoft.docs.ligo.org/ligo.skymap/ligo/skymap/plot/allsky.html#module-ligo.skymap.plot.allsky
(line 21) ok https://lscsoft.docs.ligo.org/ligo.skymap/quickstart/bayestar-injections.html
(line 7) ok https://lscsoft.docs.ligo.org/ligo.skymap/_images/ligo_skymap_plot_airmass-1.png
(line 26) ok https://lscsoft.docs.ligo.org/ligo.skymap/ligo/skymap/tool/ligo_skymap_from_samples.html
(line 7) ok https://lscsoft.docs.ligo.org/ligo.skymap/_images/allsky-1.png

build finished with problems, 3 warnings.
WARNING: Not copying tabs assets! Not compatible with linkcheck builder
ERROR: Job failed: exit code 1
failed
spelling Build
writing output... [100%] tutorial/tools
tutorial/tools.rst:15:airmass:

Spelling checker messages written to /builds/emfollow/userguide/_build/spelling/output.txt
WARNING: Found 52 misspelled words
build succeeded, 4 warnings.
Copying tabs assets
$ test ! -s _build/spelling/output.txt
ERROR: Job failed: exit code 1