Fix incorrect max upload file size on forums

The file size is calculated and displayed as a number of bytes, but was
being incorrectly labeled as 'K' [0].

The size is now correctly labeled as "bytes" and the sentence now begins
with a leading captial letter.

Impact: This has been broken on master since 2012 [1].

[0] https://github.com/edx/edx-platform/blob/master/lms/djangoapps/django_comment_client/settings.py#L4
[1] https://github.com/edx/edx-platform/commit/5d90366917c6e81513e26ea4cb8305df0f3e6d97