Fix name of module parameter in man page

[acranox]

The ZFS module parameter name is zfs_prefetch_disable, not zfs_disable_prefetch.

Motivation and Context

The man page mentioned a module parameter that is not correct.

Description

Change the name of the ZFS module parameter in the man page from zfs_disable_prefetch to the actual parameter name of zfs_prefetch_disable. This was introduced in #15436

How Has This Been Tested?

Documentation update.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• [ x ] Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• [ x ] I have updated the documentation accordingly.
• [ x ] I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• [ x ] All commit messages are properly formatted and contain Signed-off-by.