Replaced another mention of 79 characters.

CONTRIBUTING.md

@@ -7,7 +7,7 @@ should follow. These are as following:
 * When changing code make sure to write RSpec tests for the changes.
 * Document code using YARD. At the very least the method arguments and return
   value(s) should be documented.
-* Wrap lines at 79 characters per line.
+* Wrap lines at 80 characters per line.
 * Git commits should have a <= 50 character summary, optionally followed by a
   blank line and a more in depth description of 80 characters per line.