Add conversion and url to spark_functions.rst (#9876)

Summary:
Add conversion and url in spark_functions.rst to display them in Spark
functions documentation.

Pull Request resolved: #9876

Reviewed By: bikramSingh91

Differential Revision: D57921146

Pulled By: pedroerp

fbshipit-source-id: 0b5bdd55c13ae4bc96a5faec7a4ef20bbacd6406

velox/docs/spark_functions.rst

@@ -18,6 +18,8 @@ Spark Functions
     functions/spark/binary
     functions/spark/aggregate
     functions/spark/window
+    functions/spark/conversion
+    functions/spark/url
 
 Here is a list of all scalar and aggregate Spark functions available in Velox.
 Function names link to function descriptions. Check out coverage maps