Correct the output path for the translation update step.

[freakboy3742]

#310 reworked the Sphinx targets; however, I missed that the locale output directory was no longer correct as part of the changes (it was outputting into the project root directory, rather than the docs directory, due to the removal of the change_dir instruction).

This manifested as a build failure on the Update Translations job when the PR was merged.

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[rmartin16]

ah, my bad; I didn't realize that command had an implicit output directory.

[freakboy3742]

ah, my bad; I didn't realize that command had an implicit output directory.

Hey - I didn't pick it up in review either :-)