[SPARK] Remove unsupported configuration 'spark.mongodb.read.readPreference.name' (#234)

Author: rachel-mack

source/includes/connection-read-config.rst

@@ -15,8 +15,7 @@ specify them individually instead:
 
    spark.mongodb.read.connection.uri=mongodb://127.0.0.1/
    spark.mongodb.read.database=myDB
-   spark.mongodb.read.collection=myCollection
-   spark.mongodb.read.readPreference.name=primaryPreferred
+   spark.mongodb.read.collection=myCollection 
 
 .. important::