dr: alias streaming replication to PCR

Add an alias in the original Streaming Replication Between Clusters RFC to PCR,
for easier discovery.

Epic: none
Release note: None

Author: ajstorm

docs/RFCS/20201119_streaming_cluster_to_cluster.md

@@ -5,7 +5,7 @@
 - RFC PR: 56932
 - Cockroach Issue: #57433
 
-# Streaming Replication Between Clusters
+# Streaming Replication Between Clusters (aka Physical Cluster Replication - PCR)
 
 ## Summary