Docs: Update TF-M repository reference

The TF-M repository has been moved from
  git.trustedfirmware.org/trusted-firmware-m to
  git.trustedfirmware.org/TF-M/trusted-firmware-m

This patch updates references to the documentation
to reflect that change.

Signed-off-by: Minos Galanakis <minos.galanakis@arm.com>

Change-Id: Ib7d4d45bae7790bf69fcad4fd16f558716b20d92
diff --git a/docs/readme.rst b/docs/readme.rst
index 1787fb8..51657c5 100644
--- a/docs/readme.rst
+++ b/docs/readme.rst
@@ -63,7 +63,7 @@
 Really getting started
 **********************
 Trusted Firmware M source code is available on
-`git.trustedfirmware.org <https://git.trustedfirmware.org/trusted-firmware-m.git/>`__.
+`git.trustedfirmware.org <https://git.trustedfirmware.org/TF-M/trusted-firmware-m.git/>`__.
 
 To build & run TF-M: