Changed directory names to reflect actual names 📝

[triplingual]

The help dox currently say (in https://omeka.org/s/docs/developer/themes/theme_templates/) that theme template files live in "view/common/page_template or view/common/block_template" but looking at the directory structure of the Default theme, the folder names are page-template and block-template instead.

[zerocrates]

Thanks. The original docs here were written before we'd nailed down the final locations for those folders.