fixing typo in nested-aggregation.asciidoc (#26481)

docs/reference/aggregations/bucket/nested-aggregation.asciidoc

@@ -3,7 +3,7 @@
 
 A special single bucket aggregation that enables aggregating nested documents.
 
-For example, lets say we have a index of products, and each product holds the list of resellers - each having its own
+For example, lets say we have an index of products, and each product holds the list of resellers - each having its own
 price for the product. The mapping could look like:
 
 [source,js]