doc: Grammar fix.

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

@@ -9,7 +9,7 @@ Storing Sensitive Documents
 stick can know that there is a persistent storage on it. Take into consideration
 that you can be forced or tricked to give out its passphrase.
 
-Note also that **secure deletion do not work as expected on USB sticks.**<br/>
+Note also that **secure deletion does not work as expected on USB sticks.**<br/>
 [[See the corresponding documentation.|encryption_and_privacy/secure_deletion#index2h1]]
 Read also how to [[delete the persistent storage|delete]].