[docs] Update data model description

Author: mpsonntag

docs/data_model.md

@@ -1,19 +1,19 @@
-# odml data model
+# The odML data model
 
-Data exchange requires that also annoations, metadata, are
-exchanged. In oder to allow interoperability we need both a common
+Data exchange requires that also annotations, metadata, are
+shared. In oder to allow interoperability both a common
 (meta) data model, the format in which the metadata are exchanged, and
-a common terminology.
+a common terminology are required.
 
 Here, we briefly describe the *odML* data model. It is based on
 the idea of key-value pairs like ``temperature = 26°C``.
 
 The model is as simple as possible while being flexible, allowing
-interoperability, and being customizable. The model defines four
-entities (Property, Section, Value, RootSection) whose relations and
+interoperability, and being customizable. The model defines three
+entities (Property, Section, Document) whose relations and
 elements are shown in the figure below.
 
-![odml_logo](images/erModel.png "odml data model")
+![odml_logo](images/erModel.png "odML data model")
 
 Property and Section are the core entities. A Section contains
 Properties and can further have subsection thus building a tree-like