update spec document according to the reviews

Author: Yoonjae Park

SPEC.md

@@ -77,8 +77,8 @@ This section lists the space of available actions as well as their schema.
 
 To create a Hyperspace Index, specify a `DataFrame` along with index configurations. 
 `indexedColumns` are the column names used for join or filter operations. 
-Some
-`includedColumns` are the ones used for project operations.
+Some index types such as Covering Index use  
+`includedColumns` as the ones utilized for project operations.
 
 
 ### Refresh