README.rst: Avoid using non-ascii character
README.rst: Avoid using non-ascii character

Replace U+2019 with ascii 0x27.

Change-Id: I16a1d900ebc708be5d958713f1cd8364fe0d9793

jimbobhickville/taskflow,junneyang/taskflow,pombredanne/taskflow-1,openstack/taskflow,openstack/taskflow,junneyang/taskflow,pombredanne/taskflow-1,jimbobhickville/taskflow