examples: Use /bin/sh instead of bash

This changes the shell for 3 single-line invocations to the
POSIX-specified /bin/sh, because /bin/bash does not exist on a number
of systems.

examples/rtl_433_custom.php

@@ -19,7 +19,7 @@ Restart=always
 RestartSec=5
 RemainAfterExit=no
 User=root
-ExecStart=/bin/bash -c "/usr/bin/rtl_433 -f 433920000 -f 433920000 -H 120 -F syslog:127.0.0.1:1433"
+ExecStart=/bin/sh -c "/usr/bin/rtl_433 -f 433920000 -f 433920000 -H 120 -F syslog:127.0.0.1:1433"
 
 [Install]
 WantedBy=multi-user.target

examples/sigrok-conv.sh

@@ -1,3 +1,3 @@
-#!/bin/bash
+#!/bin/sh
 
 echo 'Please use "rtl_433 [-s <samplerate>] -w <output>.sr -r <input>.cu8"'

examples/sigrok-open.sh

@@ -1,3 +1,3 @@
-#!/bin/bash
+#!/bin/sh
 
 echo 'Please use "rtl_433 [-s <samplerate>] -W <output>.sr -r <input>.cu8"'