Ruby ri Problem In Windows (Solved)

I tried ri in windows 8
Console> ri String#upcase
It returned error message about not having such function.
Solution is found on Stack Overflow
SOLUTION

gem install rdoc-data

# Regenerate system docs
rdoc-data --install

# Regenerate all gem docs
gem rdoc --all --overwrite

# Regenerate specific gem doc with specific version
gem rdoc gemname -v 1.2.3 --overwrite