replace shebang '/bin/bash' for '/usr/bin/env bash'

Allows better portability of bash scripting between Linux
distributions.

Change-Id: Ia0ec079581398bcffbf20bb8206860bcec0e6e7a
Signed-off-by: Leonardo Sandoval <leonardo.sandoval@linaro.org>
diff --git a/group/tf-l1-check-docs/build-docs:nil b/group/tf-l1-check-docs/build-docs:nil
index 904c1fb..8354760 100644
--- a/group/tf-l1-check-docs/build-docs:nil
+++ b/group/tf-l1-check-docs/build-docs:nil
@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 #
 # Copyright (c) 2020, Arm Limited. All rights reserved.
 #