Change certification practices to certification practice

[pgporada]

Several years ago we changed from the plural Certification Practices Statement to the singular Certification Practice Statement. This PR makes us consistent with that. I've only updated the keyword in the translation files, not the actual translated text. That will need further review but I'm not sure of the process because there's no lastmod date in those files.

For posterity:

grep -rl Practices | xargs sed -i 's/Practices /Practice /g'
grep -rl practices | xargs sed -i 's/practices_s/practice_s/g