[1.7.x] Corrected content_type parameter name in sitemap docs.
[1.7.x] Corrected content_type parameter name in sitemap docs.

mimetype was deprecated in 1.5 per 11ec0253ab4cc926ab9e77619132cb398231ac33.

Backport of a7443c24a3 from master

oscaro/django,jyotsna1820/django,oscaro/django,jyotsna1820/django,oscaro/django,oscaro/django,jyotsna1820/django,jyotsna1820/django