Set DNS listening mode to 'ALL' in docker-compose.yml example

[cornshakes]

Description

The code snippet in README.md contains this environment variable:

FTLCONF_dns_listeningMode: 'all'

This was changed to

FTLCONF_dns_listeningMode: 'ALL'

to reflect the current documentation.

Motivation and Context

When using lower case 'all', my pi-hole still only picked up local dns requests.
Changing it to 'ALL' made it accept dns requests from other clients on the network too.

How Has This Been Tested?

I made requests from different clients on the network. At first, they were not shown in the query log and not resolved by pi-hole. After the change, all the requests appeared in the query log and were resolved.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.