Fix section formatting (#324)

* Fix level of sections

* Fix syntax of Namespace declaration example

Author: atextor

documentation/modules/ROOT/pages/modeling-guidelines.adoc

@@ -105,7 +105,7 @@ descriptions to it.
 
 [source,turtle,subs="attributes+,+quotes"]
 ----
-@prefix : urn:samm:{example-ns}.myapplication:1.0.0# .
+@prefix : <urn:samm:{example-ns}.myapplication:1.0.0#> .
 @prefix samm: <{samm}> .
 
 # The colon denotes the namespace as defined in the first @prefix declaration above.
@@ -641,7 +641,7 @@ include::example$extending-entity-declaration-inheritance.ttl[tags=extending-ent
 ----
 
 [[declaring-abstract-entities]]
-==== Declaring Abstract Entities
+=== Declaring Abstract Entities
 
 In contrast to Entities, Abstract Entities cannot directly be instantiated. A Characteristic class may have a type `samm:entity` or `samm:AbstractEntity`. But a Characteristic instance cannot have a type `samm:AbstractEntity`. If there is an instance created from a Characteristic that uses an Abstract Entity as its type, this Characteristic instance's dataType must be a sub-Entity of the Abstract Entity.
 The main use case for Abstract Entities is to share Properties across multiple Entities. This can be achieved through inheritance: Abstract Entities can extend other Entities and Abstract Entities and can themselves be extended in a manner not dissimilar to how abstract classes are used in many programming languages.
@@ -655,7 +655,7 @@ include::example$aspect-sample.ttl[tags=abstract-entity-declaration]
 ----
 
 [[abstract-entities-with-abstract-properties]]
-==== Abstract Entities with Abstract Properties
+=== Abstract Entities with Abstract Properties
 
 When an Abstract Entity needs to be defined with a certain Property, but the modeler using the
 Abstract Entity (i.e., extending it) should be able to decide the Property's Characteristic, the