Update translations build command

docs/localization_howto.md

@@ -18,7 +18,7 @@
 5. Edit the created **PO** file with your favorite editor
 6. Add the new language in the **optionsframe.py** file
 7. Build the binary translation files (MO) using the **setup.py** script:
-`python setup.py build`
+`python setup.py build_trans`
 8. Test the translations by running youtube-dl-gui:
 `python -m youtube_dl_gui`
 9. Push your changes: