Fix Wrong Unit on Slider for Images

[jsub1]

Fix for #2132. The issue was that we had to flip the index when looking up the string for the unit.

[codecov]

Codecov Report

Merging #2159 into master will increase coverage by 0.00%.
The diff coverage is 84.14%.

@@           Coverage Diff           @@
##           master    #2159   +/-   ##
=======================================
  Coverage   87.86%   87.86%           
=======================================
  Files         246      246           
  Lines       22708    22709    +1     
=======================================
+ Hits        19952    19954    +2     
+ Misses       2756     2755    -1     

Impacted Files	Coverage Δ
glue/viewers/matplotlib/state.py	90.18% <79.54%> (ø)
glue/viewers/matplotlib/viewer.py	93.92% <85.71%> (ø)
glue/viewers/histogram/qt/options_widget.py	87.09% <100.00%> (ø)
glue/viewers/image/qt/options_widget.py	84.00% <100.00%> (ø)
glue/viewers/image/qt/slice_widget.py	72.63% <100.00%> (+0.29%)	⬆️
glue/viewers/profile/qt/options_widget.py	100.00% <100.00%> (ø)
glue/viewers/scatter/qt/options_widget.py	89.74% <100.00%> (ø)
glue/utils/qt/helpers.py	55.05% <0.00%> (+1.12%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 7fcd828...3579140. Read the comment docs.

[astrofrog]

Oops thanks!