Polish fpca regression example

[Ddelval]

Several issues have been addressed:

• Fix the labels of the cross validation plot
• Change cross validation plot style
• Round scores to four decimal places

[Ddelval]

Blocked until tests are fixed ( #553 )

[vnmabus]

Please store the results in a variable before printing (do not make f-strings that complicated).

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (680fc1e) 85.86% compared to head (9c19d28) 85.86%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #552   +/-   ##
========================================
  Coverage    85.86%   85.86%           
========================================
  Files          147      147           
  Lines        11649    11649           
========================================
  Hits         10002    10002           
  Misses        1647     1647           

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