Fix _id.bookId to bookId in reviews collection references

The reviews collection has bookId as a top-level field, not nested
under _id. Fixed references in:
- $group challenge (average book rating example)
- $lookup challenge (JavaScript, mongosh, and Python examples)

Author: Copilot

docs/50-aggregation/4-group.mdx

@@ -140,7 +140,7 @@ GROUP BY year;
         await reviews.aggregate([
           {
             $group: {
-              _id: "$_id.bookId",
+              _id: "$bookId",
               avgRating: { $avg: "$rating" }
             }
           }

docs/50-aggregation/5-lookup.mdx

@@ -108,7 +108,7 @@ The $lookup operation creates an array within each book document. Using $unwind
             {
               from: "reviews",
               localField: "_id",
-              foreignField: "_id.bookId",
+              foreignField: "bookId",
               as: "reviews"
             }
           }
@@ -125,7 +125,7 @@ The $lookup operation creates an array within each book document. Using $unwind
             {
               from: "reviews",
               localField: "_id",
-              foreignField: "_id.bookId",
+              foreignField: "bookId",
               as: "reviews"
             }
           }
@@ -157,7 +157,7 @@ The $lookup operation creates an array within each book document. Using $unwind
             {
               "from": "reviews",
               "localField": "_id",
-              "foreignField": "_id.bookId",
+              "foreignField": "bookId",
               "as": "reviews"
             }
           }