docs(create-source-plugin.md): fix sentence (#11677)

Author: machour

docs/docs/create-source-plugin.md

@@ -128,7 +128,7 @@ _Examples_
 
 An example of a foreign-key relationship would be a Post that has an Author.
 
-In this relationship, each object is a distinct entity that exists whether or not the other does, with independent schemas, and field(s) on each entity that reference the other entity -- in this case the Post would have an Author, and the Author might have Posts. The API of a service that allows complex object modelling, for example a CMS, will often allow users to relationships between entities and expose them through the API.
+In this relationship, each object is a distinct entity that exists whether or not the other does, with independent schemas, and field(s) on each entity that reference the other entity -- in this case the Post would have an Author, and the Author might have Posts. The API of a service that allows complex object modelling, for example a CMS, will often allow users to add relationships between entities and expose them through the API.
 
 When an object node is deleted, Gatsby _does not_ delete any referenced entities. When using foreign-key references, it's a source plugin's responsibility to clean up any dangling entity references.