rewording

source/legacy.txt

@@ -52,7 +52,7 @@ or later. Upgrade to the current API to use the following features:
 - **Multi-document ACID transactions**, which ensure atomicity of reads and writes to multiple 
   documents and allow for transactions across multiple operations, collections, databases, 
   documents, and shards
-- **Time series collections**, which store sequences of measurements over a period 
+- **Time series collections**, or collections that store sequences of measurements over a period 
   of time and improve query efficiency for time series data
 - **Queryable Encryption**, which allows you to encrypt sensitive workloads and to query 
   the encrypted data