ITS#9200 - Fix typo in prev commit

doc/guide/admin/appendix-upgrading.sdf

@@ -64,7 +64,7 @@ of these backends must migrate their configurations to use {{slapd-mdb}}(5) prio
 H2: mdb backend
 
 It is advised to determine if the new {{slapd-mdb}}(5) idlexp backend directive and/or
-the multival database directive should be added to the OpenLDAP 2.5 configuration as well as the exising global
+the multival database directive should be added to the OpenLDAP 2.5 configuration as well as the existing global
 sortvals directive. Configuring any of these items requires that existing databases be reloaded for them to take full
 effect. This can be done separately from the overall upgrade from OpenLDAP 2.4 to OpenLDAP 2.5 if desired.