dovecot: fix index timeout seconds

data/conf/dovecot/conf.d/fts.conf

@@ -21,7 +21,7 @@ plugin {
     fts_filters = normalizer-icu snowball stopwords
     fts_filters_en = lowercase snowball english-possessive stopwords
 
-    fts_index_timeout = 300
+    fts_index_timeout = 300s
 }
 
 ### THIS PART WILL BE CHANGED BY MODIFYING mailcow.conf AUTOMATICALLY DURING RUNTIME! ###