[Fix] Fix wrong palette value in vaihingen

[MengzhangLI]

Motivation

In original vaihingen dataset pr: #1171.

The value of palette is wrong, which should be the same like Potsdam because those two datasets have same class names.

[MengzhangLI]

Note: This PR may have conflict with iSAID dataset pr.

[codecov]

Codecov Report

Merging #1292 (370d216) into master (0e3d1b8) will not change coverage.
The diff coverage is 50.00%.

@@           Coverage Diff           @@
##           master    #1292   +/-   ##
=======================================
  Coverage   90.22%   90.22%           
=======================================
  Files         130      130           
  Lines        7560     7560           
  Branches     1258     1258           
=======================================
  Hits         6821     6821           
  Misses        531      531           
  Partials      208      208           

Flag	Coverage Δ
unittests	90.22% <50.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
mmseg/core/evaluation/class_names.py	90.38% <0.00%> (ø)
mmseg/datasets/isprs.py	100.00% <100.00%> (ø)

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