Fixed path to 404 file when using custom.php (#4488)

Signed-off-by: Computroniks <mnickson@sidingsmedia.com>

automated install/basic-install.sh

@@ -1334,7 +1334,7 @@ installConfigs() {
         chmod 644 /etc/lighttpd/external.conf
         # If there is a custom block page in the html/pihole directory, replace 404 handler in lighttpd config
         if [[ -f "${PI_HOLE_BLOCKPAGE_DIR}/custom.php" ]]; then
-            sed -i 's/^\(server\.error-handler-404\s*=\s*\).*$/\1"pihole\/custom\.php"/' "${lighttpdConfig}"
+            sed -i 's/^\(server\.error-handler-404\s*=\s*\).*$/\1"\/pihole\/custom\.php"/' "${lighttpdConfig}"
         fi
         # Make the directories if they do not exist and set the owners
         mkdir -p /run/lighttpd