sentry_error_handler_vows: Fix 'Sentr' -> 'Sentry' typo

This dates back to the creation of this vow in 44c432d2 (Error handler
vows. 100% covered now, 2013-03-28).
Merge pull request #363 from wking/sentr-sentry-typo

sentry_error_handler_vows: Fix 'Sentr' -> 'Sentry' typo