using double bracket notation

Despite the script running successfully, it produced this error:

`/usr/local/bin/gravity.sh: line 8: [: missing `]'`

prior to this fix.

gravity.sh

@@ -5,7 +5,7 @@
 # This script should only be run after you have a static IP address set on the Pi
 piholeIP="$1"
 
-if [ -n "$piholeIP"]; then
+if [[ -n "$piholeIP" ]]; then
         piholeIP=$(hostname -I|xargs)        
 fi