Update Redirects.rst, grammar/typo (#766)

remove double "the"

Author: Cosmaeus

Documentation/Concepts/Backend/SiteManagement/Redirects.rst

@@ -9,6 +9,6 @@ Redirects module (Site management)
 This backend module is only available if the optional system extension
 :composer:`typo3/cms-redirects` is installed.
 
-The the Redirects system extension has a dedicated manual in which the
+The Redirects system extension has a dedicated manual in which the
 :doc:`Usage <typo3/cms-redirects:Usage/Index#usage>` of this module is
 described.