Polina Goltsman issue 11

"old:
   Usage:  An usage is the definition of a set of data structures (data
      Kinds) that an application wants to store in the overlay.  An
      usage may also define a set of network protocols (application IDs)
new:
   Usage:  A usage is the definition of a set of data structures (data
      Kinds) that an application wants to store in the overlay.  A
      usage may also define a set of network protocols (application IDs)

Typo: 2x An usage -> A usage"

- As suggested.
Merge pull request #213 from petithug/polina-goltsman-issue-11

Polina Goltsman issue 11