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
.
```

configure.ac

@@ -68,8 +68,7 @@ AC_MSG_ERROR([\
 
   You can download libInjection using git:
 
-     $ git submodule init
-     $ git submodule update
+     $ git submodule update --init --recursive
 
    ])
 fi
@@ -90,8 +89,7 @@ AC_MSG_ERROR([\
 
   You can download Mbed TLS using git:
 
-     $ git submodule init
-     $ git submodule update
+     $ git submodule update --init --recursive
 
    ])
 fi