[PostgreSQL] Update Readme File to Support Non-Multi API (#15420)

* fix readme file for multi api not supporting SDKs

* fix typo

Author: DaeunYim

specification/postgresql/resource-manager/readme.md

@@ -19,8 +19,6 @@ To see additional help and options, run:
 
 ## Configuration
 
-
-
 ### Basic Information
 These are the global settings for the PostgreSQL API.
 
@@ -31,6 +29,14 @@ openapi-type: arm
 tag: package-flexibleserver-2021-06
 ```
 
+``` yaml $(package-flexibleservers)
+tag: package-flexibleserver-2021-06
+```
+
+``` yaml $(package-singleservers)
+tag: package-2020-01-01
+```
+
 ### Tag: package-2021-06-15-privatepreview
 
 These settings apply only when `--tag=package-2021-06-15-privatepreview` is specified on the command line.