Fix NodeDriver.get_image docstring

[nightcityblade]

Fix NodeDriver.get_image docstring

Description

Correct the NodeDriver.get_image() documentation so image_id is described as an image identifier instead of a node. Also fix the malformed :rtype: field so Sphinx renders the NodeImage return type correctly.

Fixes #1540.

Status

• done, ready for review

Checklist (tick everything that applies)

• Code linting (required, can be done after the PR checks)
• Documentation
• Tests
• ICLA (required for bigger changes) — N/A for this documentation-only correction.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.58%. Comparing base (99aa044) to head (1e144bb).

Additional details and impacted files

@@           Coverage Diff           @@
##            trunk    #2180   +/-   ##
=======================================
  Coverage   83.58%   83.58%           
=======================================
  Files         352      352           
  Lines       81808    81808           
  Branches     8769     8769           
=======================================
  Hits        68373    68373           
  Misses      10562    10562           
  Partials     2873     2873           

Files with missing lines	Coverage Δ
libcloud/compute/base.py	92.86% <ø> (ø)

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