Add missing trailing space in EMAIL_SUBJECT_PREFIX

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Description

A missing space at the end of EMAIL_SUBJECT_PREFIX.

Checklist:

• I've made sure that tests are updated accordingly (especially if adding or updating a template option)
• I've updated the documentation or confirm that my change doesn't require any updates

Rationale

It doesn't look good when the EMAIL_SUBJECT_PREFIX is displayed with the subject without a space.

The documentation also contains the following note:

"You’ll probably want to include the trailing space."