Description of problem:
Typo in prerequisites regarding the firewall command to expose metrics in iptables

Version-Release number of selected component (if applicable):
all, including OCP

How reproducible:
All of the time

Steps to Reproduce:
1. Navigate to any documentation page such as: https://docs.openshift.com/container-platform/3.9/install_config/install/prerequisites.html
2.Search for the command `iptables OS_FIREWALL_ALLOW -p tcp -m state --state NEW -m tcp \
    --dport 1936 -j ACCEPT`
3. Run the command and it fails. 

Actual results:
The command fails. 

Expected results:
The command to run correctly. 

Additional info:
The command is missing a `-A` and should be: 
`iptables -A OS_FIREWALL_ALLOW -p tcp -m state --state NEW -m tcp \
    --dport 1936 -j ACCEPT`


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