doc: add --recurse-submodules when cloning the repo

User could run syscalls testing suite with
`./kirk -f ltp -r syscalls`,
but they may not be aware of the submodule `kirk`.
So I add the `--recurse-submodules` option.

Link: https://lore.kernel.org/ltp/[email protected]/
Reviewed-by: Cyril Hrubis <[email protected]>
Reviewed-by: Petr Vorel <[email protected]>
Signed-off-by: Jingyi Song <[email protected]>

doc/users/quick_start.rst

@@ -17,7 +17,7 @@ Basics requirements to build LTP are the following:
 
 .. code-block:: console
 
-   $ git clone https://github.com/linux-test-project/ltp.git
+   $ git clone --recurse-submodules https://github.com/linux-test-project/ltp.git
    $ cd ltp
    $ make autotools
    $ ./configure