Polish migration doc

Issue gh-12675

docs/modules/ROOT/pages/migration/index.adoc

@@ -4,12 +4,11 @@
 The Spring Security team has prepared the 5.8 release to simplify upgrading to Spring Security 6.0.
 Use 5.8 and the steps below to minimize changes when
 ifdef::spring-security-version[]
-xref:6.0.0@migration/index.adoc[updating to 6.0]
+xref:6.0.0@migration/index.adoc[updating to 6.0].
 endif::[]
 ifndef::spring-security-version[]
-updating to 6.0
+updating to 6.0.
 endif::[]
-.
 
 == Update to Spring Security 5.8