Fixing README instructions

It appears that mbedtls has a submodule so
git submodule update --init needs to have the --recursive flag

Test: follow the old setup instructions, then follow the new ones
Change-Id: I42c9de1a2bedad3c43142ec55f0b65ed1b9148d8
Reviewed-on: https://pigweed-review.googlesource.com/c/open-dice/+/260257
Reviewed-by: Darren Krahn <dkrahn@google.com>
Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>
Commit-Queue: Darren Krahn <dkrahn@google.com>
diff --git a/README.md b/README.md
index 27806c9..3e78434 100644
--- a/README.md
+++ b/README.md
@@ -54,7 +54,7 @@
 To setup the build environment the first time:
 
 ```bash
-$ git submodule update --init
+$ git submodule update --init --recursive
 $ source bootstrap.sh
 $ gn gen out
 ```