Update db.collection.countDocuments.txt

Removed an extra closing parenthesis

Author: ryanmercadante

source/reference/method/db.collection.countDocuments.txt

@@ -95,7 +95,7 @@ aggregation operation and returns just the value of ``n``:
 
    db.collection.aggregate([
       { $match: <query> },
-      { $group: { _id: null, n: { $sum: 1 } } } )
+      { $group: { _id: null, n: { $sum: 1 } } }
    ])
 
 Empty or Non-Existing Collections and Views