Correct one spelling error

[randolf]

I'm correcting a spelling error that I encountered while reading the documentation.

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You are welcome to open PR, but they are used for discussion only. All patches must eventually go to the openvpn-devel mailing list for review:

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• https://community.openvpn.net/openvpn/wiki/DeveloperDocumentation
• https://community.openvpn.net/openvpn/wiki/Contributing