consistent spelling

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@4372 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

doc/ffserver.conf

@@ -137,7 +137,7 @@ VideoGopSize 12
 # Set this to the number of seconds backwards in time to start. Note that
 # most players will buffer 5-10 seconds of video, and also you need to allow
 # for a keyframe to appear in the data stream.
-#PreRoll 15
+#Preroll 15
 
 # ACL: