Correct capitalization of GemFire in the reference documentation

[philwebb]

Features currently has GemFire references, we should also include Geode.

[wilkinsona]

This looks OK to me. The reference to GemFire is in an item in a bulleted list and it says "Gemfire or Geode". The capitalization of GemFire is incorrect, however. I'll use this issue to fix that.