Update docs/reference.md [no ci]

Author: colleenXu

docs/reference.md

@@ -284,7 +284,7 @@ Generic attribute for a node or an edge that expands the key-value pair concept
 
 | Field Name | Type | Description |
 | --- | :---: | --- |
-| attribute_type_id | [CURIE](#curie-) | **REQUIRED**. The primary 'key' of the attribute object, holding a CURIE of an ontology property defining the specific attribute (preferably the CURIE of a Biolink association slot). This property captures the relationship asserted to hold between the value of the attribute, and the node or edge from  which it hangs. For example, that a value of '0.000153' represents a p-value supporting an edge. |
+| attribute_type_id | [CURIE](#curie-) | **REQUIRED**. The main 'key' of the attribute object, holding a CURIE of an ontology property defining the specific attribute (preferably the CURIE of a Biolink association slot). This property captures the relationship asserted to hold between the value of the attribute, and the node or edge from  which it hangs. For example, that a value of '0.000153' represents a p-value supporting an edge. |
 | original_attribute_name | `string` | The term used by the original source of an attribute to describe the meaning or significance of the value it captures. This may be a column name in a source tsv file, or a key in a source json document for the field in the data that held the attribute's value. Capturing this information  where possible lets us preserve what the original source said. Note that the data type is string' but the contents of the property could also be a CURIE of a third party ontology term. |
 | value | any | **REQUIRED**. Value of the attribute. May be any data type, including a list. |
 | value_type_id | [CURIE](#curie-) | CURIE describing the semantic type of an  attribute's value. Use a Biolink class if possible, otherwise a term from an external ontology. If a suitable CURIE/identifier does not exist, enter a descriptive phrase here and submit the new type for consideration by the appropriate authority. |