Fix Practical MDB Aggs Book Link

Author: npentrel

source/index.txt

@@ -156,7 +156,7 @@ MongoDB. You can:
   <https://www.mongodb.com/lp/whitepaper/architecture-guide?jmp=docs>`_
 
 - Read the `Practical MongoDB Aggregations
-  <https://www.practical-mongodb-aggregations.com>` e-book__
+  <https://www.practical-mongodb-aggregations.com>`__ e-book
 
 Licensing
 ---------