Fixed invalid HTML in upgrades documentation.
Fixed invalid HTML in upgrades documentation.

Review: https://reviews.apache.org/r/45242/

neilconway/mesos,reneploetz/mesos,craimbert/mesos,asamerh4/mesos,jpeach/mesos,asamerh4/mesos,reneploetz/mesos,andschwa/mesos,jpeach/mesos,yuquanshan/customizedMesos,dforsyth/mesos,kaysoky/mesos,yuquanshan/customizedMesos,neilconway/mesos,Aman-Jain-14/customizedMesos,abudnik/mesos,yuquanshan/customizedMesos,craimbert/mesos,neilconway/mesos,kaysoky/mesos,asamerh4/mesos,gsantovena/mesos,yuquanshan/customizedMesos,Gilbert88/mesos,zmalik/mesos,andschwa/mesos,Gilbert88/mesos,Gilbert88/mesos,craimbert/mesos,reneploetz/mesos,reneploetz/mesos,asamerh4/mesos,Gilbert88/mesos,jpeach/mesos,zmalik/mesos,Aman-Jain-14/customizedMesos,kaysoky/mesos,chhsia0/mesos,dforsyth/mesos,chhsia0/mesos,craimbert/mesos,zmalik/mesos,gsantovena/mesos,Gilbert88/mesos,gsantovena/mesos,craimbert/mesos,abudnik/mesos,yuquanshan/customizedMesos,Aman-Jain-14/customizedMesos,gsantovena/mesos,Gilbert88/mesos,Aman-Jain-14/customizedMesos,kaysoky/mesos,asamerh4/mesos,jpeach/mesos,zmalik/mesos,dforsyth/mesos,verizonlabs/mesos,jpeach/mesos,andschwa/mesos,craimbert/mesos,yuquanshan/customizedMesos,asamerh4/mesos,andschwa/mesos,verizonlabs/mesos,asamerh4/mesos,craimbert/mesos,zmalik/mesos,chhsia0/mesos,zmalik/mesos,reneploetz/mesos,verizonlabs/mesos,neilconway/mesos,neilconway/mesos,abudnik/mesos,Aman-Jain-14/customizedMesos,andschwa/mesos,dforsyth/mesos,kaysoky/mesos,gsantovena/mesos,andschwa/mesos,yuquanshan/customizedMesos,andschwa/mesos,dforsyth/mesos,reneploetz/mesos,reneploetz/mesos,abudnik/mesos,Aman-Jain-14/customizedMesos,kaysoky/mesos,chhsia0/mesos,jpeach/mesos,chhsia0/mesos,chhsia0/mesos,Gilbert88/mesos,gsantovena/mesos,dforsyth/mesos,neilconway/mesos,gsantovena/mesos,zmalik/mesos,dforsyth/mesos,Aman-Jain-14/customizedMesos,verizonlabs/mesos,abudnik/mesos,chhsia0/mesos,kaysoky/mesos,verizonlabs/mesos,abudnik/mesos,abudnik/mesos,verizonlabs/mesos,jpeach/mesos,neilconway/mesos,verizonlabs/mesos