Bug 1260656 - Fix firefox-puppeteer documentation generation for Readthedocs. r=maja_zf

MozReview-Commit-ID: 8CmozumFEmO

--HG--
extra : rebase_source : bee3eb8f232fff90010746e388cbbdf1cf2970ad

testing/puppeteer/firefox/requirements-docs.txt

@@ -2,4 +2,5 @@ sphinx
 sphinx_rtd_theme
 
 # Required by Readthedocs to install the firefox-puppeteer package
--e .
+-e testing/puppeteer/firefox
+