update pipeline.asciidoc

typo

docs/reference/aggregations/pipeline.asciidoc

@@ -187,7 +187,7 @@ POST /_search
 <1> By using `_count` instead of a metric name, we can calculate the derivative of document counts in the histogram
 
 The `buckets_path` can also use `"_bucket_count"` and path to a multi-bucket aggregation to use the number of buckets
-returned by that aggregation in the pipeline aggregation instead of a metric. for example a `bucket_selector` can be
+returned by that aggregation in the pipeline aggregation instead of a metric. For example, a `bucket_selector` can be
 used here to filter out buckets which contain no buckets for an inner terms aggregation:
 
 [source,console,id=buckets-path-bucket-count-example]