playbook: Update README.md

playbook/roles/install-script/README.md

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 The installation scripts are splited on 4 diferent keys or channels:
 
-  slapos-global-key: Fully stable playbook, Dedicated to KVM cluster autosetup images and
+- `slapos-global-key` Fully stable playbook, Dedicated to KVM cluster autosetup images and 
     computers who host KVM clusters on IMT infraestructure.
 
-  slapos-global-unstable-key: Unstable or Development playbook alternative for 
+- `slapos-global-unstable-key` Unstable or Development playbook alternative for 
     slapos-global-key.
 
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-  slapos-global-vifib-official-key: Official VIFIB and Grandenet Stable Playbook, it include
+- `slapos-global-vifib-official-key` Official VIFIB and Grandenet Stable Playbook, it include
     either stable versions of the Cluster/Standalone servers. 
              
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-  slapos-global-vifib-testing-key: Testing Playbook VIFIB and Grandenet Stable Playbook, it 
+- `slapos-global-vifib-testing-key` Testing Playbook VIFIB and Grandenet Stable Playbook, it 
     include unstable (or more recently versions) of the Cluster/Standalone servers.