The variable 'celsius' was spelled 'Celsius' for no apparent reason in the novice R material.  This PR makes it consistently lower-case.
Fixed merge conflict due to mixing of tabs/spaces in PR #490 from @gvwilson. Coverted spelling of Celsius varialbe to lowercase in function kelvin_to_celsius.
Merge pull request #491 from jdblischak/lowercasing-celsius-in-r

Lowercasing celsius in r