Fix typo in filters aggregation docs (#21690)

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

@@ -107,7 +107,7 @@ request.  The response for this example would be:
         {
           "doc_count" : 34,
           "monthly" : {
-            "buckets : [
+            "buckets" : [
               ... // the histogram monthly breakdown
             ]
           }
@@ -115,7 +115,7 @@ request.  The response for this example would be:
         {
           "doc_count" : 439,
           "monthly" : {
-            "buckets : [
+            "buckets" : [
               ... // the histogram monthly breakdown
             ]
           }