Update rake task for creating a blog post

Why is this change being made?

To create a blog post, you can run bundle exec rake new_post, as per the instructions in the docs. This MR updates the directory location where the file is created to -> sites/blog/source/blog/blog-posts, since it was previously being created in the wrong directory.

Does this MR meet the acceptance criteria?

Assign to DRI

• Did you assign this change to the correct DRI of the page or information you are changing?

Conformity

• Added description to this MR explaining the reasons for the proposed change, per say-why-not-just-what