Fix redirects from guide to opsmanual
Fix redirects from guide to opsmanual

https://github.com/alphagov/govuk-developer-docs/pull/73 broke the
site, because contrary to middleman's readme we shouldn't be using
leading slashes.

alphagov/govuk-developer-docs,alphagov/govuk-developer-docs,alphagov/govuk-developer-docs,alphagov/govuk-developer-docs