Fix volume path for PostgreSQL in develop.md

When running   docker compose up --build 
this happens:
...
Attaching to db-1, server-1
db-1  | Error: in 18+, these Docker images are configured to store database data in a
db-1  |        format which is compatible with "pg_ctlcluster" (specifically, using
db-1  |        major-version-specific directory names).  This better reflects how
db-1  |        PostgreSQL itself works, and how upgrades are to be performed.
db-1  | 
db-1  |        See also docker-library/postgres#1259
...

Author: dagett

content/guides/python/develop.md

@@ -211,7 +211,7 @@ You'll need to clone a new repository to get a sample application that includes
    #     secrets:
    #       - db-password
    #     volumes:
-   #       - db-data:/var/lib/postgresql/data
+   #       -db-data:/var/lib/postgresql
    #     environment:
    #       - POSTGRES_DB=example
    #       - POSTGRES_PASSWORD_FILE=/run/secrets/db-password
@@ -364,7 +364,7 @@ services:
     secrets:
       - db-password
     volumes:
-      - db-data:/var/lib/postgresql/data
+      -db-data:/var/lib/postgresql
     environment:
       - POSTGRES_DB=example
       - POSTGRES_PASSWORD_FILE=/run/secrets/db-password
@@ -507,7 +507,7 @@ services:
     secrets:
       - db-password
     volumes:
-      - db-data:/var/lib/postgresql/data
+      -db-data:/var/lib/postgresql
     environment:
       - POSTGRES_DB=example
       - POSTGRES_PASSWORD_FILE=/run/secrets/db-password