README: adjust for final Azure Pipeline ID

During the six months of development of the Azure Pipelines support, the
patches went through quite a few iterations of changes, and to test
those iterations, a temporary build definition was used.

In the meantime, Azure Pipelines support made it to `master`, and we now
have a regular Azure Pipeline, installed via the common GitHub App
workflow. This new pipeline has a different name (git.git instead of
test-git.git), and a new ID (11 instead of 2).

Let's adjust the badge in our README to reflect that final shape of the
Azure Pipeline.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

README.md

-[![Build Status](https://dev.azure.com/git/git/_apis/build/status/test-git.git)](https://dev.azure.com/git/git/_build/latest?definitionId=2)
+[![Build Status](https://dev.azure.com/git/git/_apis/build/status/git.git)](https://dev.azure.com/git/git/_build/latest?definitionId=11)
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