add --recursive flag as per @bgould suggestion

Author: soypat

content/docs/guides/selfbuild/_index.md

@@ -26,7 +26,7 @@ git checkout dev
 Once the branch is selected, pull submodules:
 
 ```shell
-git submodule update --init
+git submodule update --init --recursive
 ```
 
 Now you are ready to build TinyGo- but you must choose whether to build with a [manual LLVM install](./manual-llvm) or with a [system installed LLVM](./bring-your-own-llvm). After building you should also read [additional requirements](./additional-requirements) to make sure you've fulfilled all the requirements for the features of TinyGo you'll be using.