Fix regexp expansion of the lurker-index command

conf/services.d/smtpd/run

 #!/bin/sh
 
 # Build the SMTP_RULES env var based on the LURKER_ADDR_RX regexp.
-export SMTP_RULES="${LURKER_ADDR_RX}:/usr/bin/lurker-index -m -l \\1"
+export SMTP_RULES="${LURKER_ADDR_RX}:/usr/bin/lurker-index -m -l \$1"
 
 # Temporarily suspend incoming email when lurker is rebuilding its index.
 export SMTP_GUARD_FILE="${LURKER_DATA_DIR:-/var/lib/lurker/data}/.tmp"