doc: Fix typos.

wiki/src/doc/first_steps/persistence.caution.mdwn

 <div class="caution">
 <strong>The use of a persistent volume in a system which is designed to provide
 anonymity and leave no trace is a complicated issue.</strong><br/>
-[[Read carefully the warnings section.|persistence/warnings]]
+[[Read carefully the warning section.|persistence/warnings]]
 </div>

wiki/src/doc/first_steps/persistence/warnings.mdwn

@@ -18,7 +18,7 @@ Overwriting Configurations
 
 The persistent volume allows you to overwrite the configuration of the programs
 included in Tails. Those programs are carefully configured with security in mind.
-Overwriting the default configuration can break this security or render them unusuable.
+Overwriting the default configuration can break this security or render them unusable.
 
 Furthermore, the anonymity of Tor and Tails relies on making it harder to
 distinguish one Tails user from another. <strong>Changing the default

wiki/src/doc/first_steps/startup_options.mdwn

 [[!meta title="Startup Options"]]
 
 When starting Tails, you can specify startup options to alter some of its basic
-functionning. The two ways of specifying startup options are the following:
+functioning. The two ways of specifying startup options are the following:
 
 [[!toc levels=1]]