[1.0.X] Fixed #9572 -- Corrected some documentation that misleadingly gave the...

[1.0.X] Fixed #9572 -- Corrected some documentation that misleadingly gave the extension for YAML fixtures as YML. Thanks to django@i-sh.me.uk for the report.

Merge from trunk of [9401]


git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.0.X@9402 bcc190cf-cafb-0310-a4f2-bffc1f526a37