[OUDS] Fix(README): composer command line

[julien-deramond]

Context

There were errors mentioned at https://packagist.org/packages/orange-opensource/orange-boosted-bootstrap and received by emails as error notifications too.
These have been supposedly fixed.

Description

This PR updates the command mentioned in our README file.

The name provided in our current ouds/main for Composer is "name": "orange-opensource/ouds-web",, targetting the future repository URL.
It means that, in the meantime, there won't be any vx.y.z-ouds-web Composer versions at https://packagist.org/packages/orange-opensource/orange-boosted-bootstrap.
We don't have many users, so the solution, for now is to provide the reference of our main OUDS Web branch.

Locally, you can test to run: composer require orange-opensource/orange-boosted-bootstrap:dev-ouds/main:

./composer.json has been created
Running composer update orange-opensource/orange-boosted-bootstrap
Loading composer repositories with package information
Updating dependencies
Lock file operations: 1 install, 0 updates, 0 removals
  - Locking orange-opensource/orange-boosted-bootstrap (dev-ouds/main 847cf7d)
Writing lock file
Installing dependencies from lock file (including require-dev)
Package operations: 1 install, 0 updates, 0 removals
  - Installing orange-opensource/orange-boosted-bootstrap (dev-ouds/main 847cf7d): Extracting archive
Generating autoload files
No security vulnerability advisories found

As you can see, it uses our current latest commit (847cf7d).

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