Change SPHINXOPTS in doc/makefile

[chan-hoo]

Description

• Although the recent change by PR Add docs-folder to git action yaml #81 fixed the failure, a warning message was found in the step upload_artifact.
• The author of sphinx-action recommended the change in this PR.

Anticipated changes to regression tests:

• Is baseline change expected ?

Subcomponents involved:

• DA_update (ufs-community/land-DA)
• ufsLand.fd (NOAA-EPIC/ufs-land-driver-emc-dev)
• ufs_model.fd (ufs-community/ufs-weather-model)
• vector2tile_converter.fd (NOAA-PSL/land-vector2tile)
• none

Linked PR's and Issues:

Fixes Issue #84

Testing (for CM's):

• RDHPCS
  • Hera
  • Orion
  • Jet
  • Gaea
  • Cheyenne
• CI
  • Completed
• PW-Clouds
  • AWS
  • AZURE
  • GCP

[chan-hoo]

@jkbk2004, please review this PR. As described above, I found a warning message in auto_build of the other PRs. The developer of sphinx-action recommended this change. I don't find the same warning in testing this PR, but I'd like to check it in other PRs once this pr is merged.

[chan-hoo]

Thanks! :)