[4.0] 9557bcf Fix typo

[Lasse Karstensen]

commit 9557bcffb2c20b1cbf939dcb1ad57d9338d663af
Author: Federico G. Schwindt <fgsch at lodoss.net>
Date:   Mon Nov 3 10:52:05 2014 +0000

    Fix typo
    
    Submitted by:	Muhammad Adeel via github

diff --git a/doc/sphinx/phk/persistent.rst b/doc/sphinx/phk/persistent.rst
index 33af07c..6c1d29c 100644
--- a/doc/sphinx/phk/persistent.rst
+++ b/doc/sphinx/phk/persistent.rst
@@ -41,7 +41,7 @@ all the banned objects.
 With persistent storage, we not only have to store the still live
 bans with the cached objects, and keep the two painfully in sync,
 so the bans gets revived with the objects, we also have to worry
-about missing bans duing the downtime, since those might ban objects
+about missing bans during the downtime, since those might ban objects
 we will recover on startup.
 
 Ouch:  Straight into database/filesystem consistency territory.