Fix config name typo in SetupCheckPygment

Summary:
Use correct spelling of 'environment.append-paths' so that the current
value of the variable will display as expected in the
'pygmentize Not Found' setup issue screen.

Test Plan:
* Enabled Pygments but haven't installed it
* Follow 'unresolved setup issues' link to 'Not Found' screen
* See that 'envinronment.append-paths' is None
* Set 'environment.append-paths'
* See that 'envinronment.append-paths' is still None
* Apply this fix
* See that 'environment.append-paths' is now '/usr/bin'

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5555