Fix install-guides build in Mac OS X
Fix install-guides build in Mac OS X

In Mac OS X environment, the actual behavior of grep command
with "-m" option is different from the expected behavior.

Instead of using "-m 1" option, adding one pipeline
with "head -n 6" (since "-A 5" looks after 5 lines more)
makes the build compatible not only Linux but also
Mac OS X environment.

Change-Id: I63fc0fd0c8afde883ffaf69c9b674c88d8079014
Closes-Bug: #1619358

openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals