ruby: 0000-fundamentals.rb: added .inject for mathematical application
ruby: 0000-fundamentals.rb: added .inject for mathematical application

This patch is created to demonstrate .inject being used for mathematical
functions. In this example, it is used to calculate sum of cube for
a range of numbers (5 to 10).

Signed-off-by: (Holloway) Chew, Kean Ho <bbd8faa40b0f348214bf4124b281a301299ba6dd@gmail.com>

hollowaykeanho/learning,hollowaykeanho/learning,josiahsoh/learning,hollowaykeanho/learning,josiahsoh/learning,josiahsoh/learning,josiahsoh/learning,hollowaykeanho/learning,hollowaykeanho/learning,josiahsoh/learning