fix typo in doc/source/image_builders/upload.rst
fix typo in doc/source/image_builders/upload.rst

fix typo in doc/source/image_builders/upload.rst
cirrus --> cirros

Change-Id: I82a448840132361885ee17b445c4fc3fcb536184

DavidPurcell/murano_temp,satish-avninetworks/murano,olivierlemasle/murano,olivierlemasle/murano,satish-avninetworks/murano,NeCTAR-RC/murano,NeCTAR-RC/murano,olivierlemasle/murano,openstack/murano,openstack/murano,satish-avninetworks/murano,satish-avninetworks/murano,DavidPurcell/murano_temp,DavidPurcell/murano_temp,olivierlemasle/murano,DavidPurcell/murano_temp,NeCTAR-RC/murano,NeCTAR-RC/murano