[Doc] Fixed some minor syntax issues

[javiereguiluz]

This fixes the following errors when building docs:

Errors for SchebTwoFactorBundle / 6.x

Found invalid reference "related packages on Packagist.org<https://packagist.org/packages/scheb/2fa-bundle/dependents>" in file "installation"

Found invalid reference "related packages on Packagist.org<https://packagist.org/packages/scheb/2fa-bundle/dependents>" in file "index"

In RST, the syntax for links is:

For links that appear more than once in the docs and that define their URL separately:

`The link text`_

.. _`The link text`: https://example.com

For one-time links that define their URL embedded:

`The link text <https://example.com>`__

CAUTION: in the second case, the link ends with 2 underscores (__) instead of just one as in the first case (_)