Update Leaders donation limit text

[house-ben]

Fixed the contribution description for #1766

Since the heroes are based on the total donated_amount contributed, not the donations over the past year, the text has to be updated. https://github.com/django/djangoproject.com/blob/100d673a8ff0a9072ee0eeebcee4252f396049bf/fundraising/templatetags/fundraising_extras.py#L95-L113

The fundraisers also have to have donated in the past year to show up https://github.com/django/djangoproject.com/blob/5537f965ece29b766efe17c1d2137c3d2037811c/fundraising/models.py#L25-L35

[house-ben]

I tried squashing my commits. I'm not familiar enough with git to understand how to get my two commits down to one. Anyone that can help?