Fix issues with prefetch override docs

[misenhower]

This fixes the following issues:

• The option is fileBlacklist, not fileBlackList (lowercase "L").
• This option is not specified by default, so you have to make sure it exists before calling push on it.
• You have to use options[0] to access the first options array member to get to the actual plugin options object (I'm not sure if this is documented better elsewhere?).