Fix typo on README.md (pgcentralfoundation#1995)

Fix small typo on postgres version

Author: lizardoluis

cargo-pgrx/README.md

@@ -236,7 +236,7 @@ $ cargo pgrx start all
     Starting Postgres v13 on port 28813
     Starting Postgres v14 on port 28814
     Starting Postgres v15 on port 28815
-    Starting Postgres v15 on port 28816
+    Starting Postgres v16 on port 28816
 
 $ cargo pgrx status all
 Postgres v12 is running