Update chapters/compatibility.adoc

Author: ePaul

chapters/compatibility.adoc

@@ -46,7 +46,7 @@ services in a backward-compatible way.
 In general:
 
 * Never change the semantic of fields (e.g. changing the semantic from
-customer-number to customer-id, as both are different unique customer keys)
+  customer-number to customer-id, as both are different unique customer keys)
 * Consider <<251>> in case a URL has to change.
 
 For schemas used in input only: