Fixed #20763 -- Typo in tutorial 3.

Thanks crichard@ for the report.
Merge pull request #1370 from Matt-Stevens/doc-url-fix

Fixed broken Atom URLs in syndication docs
Fixed #20762 -- Clarified that add_fieldsets is not a standard ModelAdmin attribute.
Cleaned up UserAdmin.get_form() that worked around a bug fixed in 23e1b59.

Refs #18681.
Merge pull request #1372 from loic/admin

Cleaned up UserAdmin.get_form() that worked around a bug fixed in 23e1b59.
Fixed #20767 -- Fixed ModelAdmin.preserve_filters for namespaced URLs.

Thanks Collin Anderson for the report.