MINOR: Fix typos in security section
MINOR: Fix typos in security section

1. I think the instructions in step 2 of the security section which describe adding the CA to server/client truststores are swapped. That is, the instruction that says to add the CA to the server truststore adds it to the client truststore (and vice versa).
2. "clients keys" should be possessive ("clients' keys").

This contribution is my original work, and I license the work to the project under the project's open source license.

Author: Samuel Taylor <staylor@square-root.com>

Reviewers: Ismael Juma <ismael@juma.me.uk>

Closes #1651 from ssaamm/trunk

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