Fix documentation workflow warning.log archive

[astrogeco]

Describe the bug
The documentation CI workflow has a typo in the directory for the usersguide-warnings.log output file.

To Reproduce
See https://github.com/nasa/cFE/runs/2847522531?check_suite_focus=true

Expected behavior
Workflow runs successfully and reports and archives warnings

Code snips

cFE/.github/workflows/build-documentation.yml

Line 116 in d03e24e

mv build/doc/warnings.log usersguide_warnings.log

Additional context
This popped up after the update to the documentation build and I missed it when fixing up the IC

[jphickey]

Just ran into this on my PR, seems like a concurrent add/update issue (this is a new-ish workflow item, right?)