doc: Fix README to update git submodules

The old instruction causes the following error:
```
$ git submodule foreach git pull
Entering 'bindings/python'
You are not currently on a branch.
Please specify which branch you want to merge with.
See git-pull(1) for details.

    git pull <remote> <branch>

fatal: run_command returned non-zero status for bindings/python
.
```

README.md

@@ -217,7 +217,7 @@ the utilities, follow the commands listed below:
 
 ```shell
 $ cd /path/to/your/ModSecurity
-$ git submodule foreach git pull
+$ git submodule update --init --recursive
 $ make check
  ```