Update battery_percentage.sh

https://github.com/tmux-plugins/tmux-battery/pull/108/files#diff-ecafee011a0ba7e9c912b4a1ab3d1e5594e6bd823eb4f81ce067fd45f0d0a08b

Author: slange-dev

scripts/battery_percentage.sh

@@ -7,9 +7,9 @@ source "$CURRENT_DIR/helpers.sh"
 print_battery_percentage() {
 	# percentage displayed in the 2nd field of the 2nd row
 	if is_wsl; then
-		local battery
-		battery=$(find /sys/class/power_supply/*/capacity | tail -n1)
-		cat "$battery"
+		if [ -e /sys/class/power_supply/*/capacity ]; then
+			echo $(cat $(find /sys/class/power_supply/*/capacity | tail -n1))%
+		fi
 	elif command_exists "pmset"; then
 		pmset -g batt | grep -o "[0-9]\{1,3\}%"
 	elif command_exists "acpi"; then