fix(docs): correct `--no-ff` -> `--ff-only`

The `--no-ff` flag always creates a merge commit; that's the opposite of
what we meant when this documentation was updated. Use `--ff-only` to
require fast-forwards pulls only.

Change-Id: Ia49ddccbfc998e403e900e9fa86f55129fe521b3
Signed-off-by: Chris Kay <chris.kay@arm.com>
diff --git a/docs/getting_started/requirements.rst b/docs/getting_started/requirements.rst
index 6b8964a..798efcf 100644
--- a/docs/getting_started/requirements.rst
+++ b/docs/getting_started/requirements.rst
@@ -42,7 +42,7 @@
 
 .. code-block:: bash
 
-   git pull --rebase --no-ff
+   git pull --rebase --ff-only
    git submodule update --init --recursive
 
 To fetch the latest commits from all submodules, you can use: