Add missing link to Spark task in docs

docs/liminal/tasks/README.md

diff --git a/docs/liminal/tasks/README.md b/docs/liminal/tasks/README.md
index 44561b8..be36fc5 100644
--- a/docs/liminal/tasks/README.md
+++ b/docs/liminal/tasks/README.md
@@ -48,5 +48,6 @@
 ## task types
 
 1. [python](python.md)
-2. [create_cloudformation_stack](create_cloudformation_stack.md)
-3. [delete_cloudformation_stack](delete_cloudformation_stack.md)
+2. [spark](spark.md)
+3. [create_cloudformation_stack](create_cloudformation_stack.md)
+4. [delete_cloudformation_stack](delete_cloudformation_stack.md)