auth_openidc.conf: fix missed spell check backrefrences

Signed-off-by: Hans Zandbelt <[email protected]>

auth_openidc.conf

@@ -819,7 +819,7 @@
 # resulting value from the 1st parameter and the first match returned from that expression will
 # be set as the REMOTE_USER. E.g. to strip a domain from an e-mail style address you'd use ^(.*)@
 #
-# An optional 3rd parameter can be added that would contain string with number backrefrences.
+# An optional 3rd parameter can be added that would contain string with number backreferences.
 # Backreferences must be in the form $1, $2.. etc.
 # E.g. to extract username in the form DOMAIN\userid from e-mail style address you may use
 #  ^(.*)@([^.]+)\..+$ $2\\$1