Fix paste-o in examine-backward.exp.

Fix paste-o in examine-backward.exp.

This patch fixes a paste-o that was introduced in commit
c8ad9b9a31aa3e6039567fc1f152dd454c946d5f.  Previously the regexp for
the "examine 3 bytes backward from ${address_zero}" test correctly
matched 3 "${byte}" patterns, but in that commit the 6-byte regexp
from the previous test was mistakenly repeated here instead.

2019-08-15  Sandra Loosemore  <sandra@codesourcery.com>

	gdb/testsuite/
	* gdb.base/examine-backward.exp: Correct regexp for
	"examine 3 bytes backward from ${address_zero}".