fix(plus): wrong shebang

sh/tests/plus_test.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 set -euo pipefail
 IFS='

sh/tests/solutions/plus.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 # Add the two numbers
 result=$(expr $1 + $2)