Update zpool-attach.8

[trick2011]

Modified all instances that refer to the existing vdev by device to vdev in zpool attach, in order to clarify usage.

Fixes bug report #17734

This commit fixes unclear usage of device when referring to the existing vdev in zpool attach manpage.

Motivation and Context

This fixes open issue #17734

When looking up the parameters to use for zpool attach, the meaning of "device" in the parameter description is unclear/ambiguous. The name "device" refers to a vdev in the pool and as such it is clearer to refer to it as such as the intent, attaching a new device to an existing vdev, is communicated more clearly this way.

Description

How Has This Been Tested?

Opened using man and visually checked

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)
• Quality assurance (non-breaking change which makes the code more robust against bugs)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• 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.
• All commit messages are properly formatted and contain Signed-off-by.