fset: fix fset.color.help_description not applying

[nathanielevan]

Description

[why]
The fset.color.help_description setting previously didn't apply to its corresponding UI element, in this case the description of configuration variable in the help bar in fset.

[how]
Change code that configures the foreground colour of this UI element to apply the aforementioned setting.

[nathanielevan]

Quick demonstration of how this looks like:

old	new

[codecov-commenter]

Codecov Report

Merging #1988 (a6cb6bd) into master (06fa592) will decrease coverage by 0.01%.
Report is 11 commits behind head on master.
The diff coverage is 20.00%.

❗ Current head a6cb6bd differs from pull request most recent head 9b838c5. Consider uploading reports for the commit 9b838c5 to get more accurate results

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@@            Coverage Diff             @@
##           master    #1988      +/-   ##
==========================================
- Coverage   47.25%   47.25%   -0.01%     
==========================================
  Files         215      215              
  Lines       90466    90468       +2     
==========================================
  Hits        42754    42754              
- Misses      47712    47714       +2     

Files Changed	Coverage Δ
src/plugins/fset/fset-bar-item.c	9.72% <0.00%> (-0.28%)	⬇️
src/plugins/irc/irc-channel.c	60.85% <0.00%> (ø)
src/gui/gui-input.c	56.98% <100.00%> (ø)

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[flashcode]

Thanks!