Fix link and update admonition (#68)

Author: Chris Cho

source/kafka-sink.txt

@@ -22,14 +22,12 @@ saves the data to a MongoDB database.
 This section of the guide covers the configuration settings necessary to
 set up a Kafka Sink connector.
 
-.. admonition:: Add Indexes to Your Collections for Consistent Performance
-   :class: important
+.. important:: Add Indexes to Your Collections for Consistent Performance
 
    Writes performed by the Kafka Sink Connector take additional
    time to complete as the size of the underlying MongoDB collection
    grows. To prevent performance deterioration,
-   :manual:`use an index to support these queries
-   <tutorial/create-indexes-to-support-queries/>`.
+   :manual:`use an index to support these queries </tutorial/create-indexes-to-support-queries/>`.
 
 
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