Get rid of @mode in #' @examplesIf for info gain

[franceslinyc]

Edit the documentation to reflect the latest changes for info gain.

Previously, user have to set object@mode <- "regression" for regression task. They don't now, and this PR reflects the changes.

[franceslinyc]

Error: 
  ! error in pak subprocess
  Caused by error in `stop_task_build(state, worker)`:
  ! Failed to build source package aorsf.
  Full installation output:
  * installing *source* package ‘aorsf’ ...
  ** this is package ‘aorsf’ version ‘0.1.5’
  ** package ‘aorsf’ successfully unpacked and MD5 sums checked
  staged installation is only possible with locking
  ** using non-staged installation
  ** libs
  using C++ compiler: ‘g++ (Ubuntu 13.3.0-6ubuntu2~24.04) 13.3.0’
  g++ -std=gnu++17 -I"/opt/R/devel/lib/R/include" -DNDEBUG  -I'/home/runner/work/_temp/Library/Rcpp/include' -I'/home/runner/work/_temp/Library/RcppArmadillo/include' -I/usr/local/include   -fopenmp -fpic  -g -O2  -Wall -pedantic  -c AAA_check_cpp_version.cpp -o AAA_check_cpp_version.o
  g++ -std=gnu++17 -I"/opt/R/devel/lib/R/include" -DNDEBUG  -I'/home/runner/work/_temp/Library/Rcpp/include' -I'/home/runner/work/_temp/Library/RcppArmadillo/include' -I/usr/local/include   -fopenmp -fpic  -g -O2  -Wall -pedantic  -c Coxph.cpp -o Coxph.o
  In file included from Coxph.cpp:9:
  Coxph.h:10:10: fatal error: armadillo: No such file or directory
     10 | #include <armadillo>
        |          ^~~~~~~~~~~
  compilation terminated.

[franceslinyc]

@EmilHvitfeldt need to bug you on this when you have a time!