Update README.md

`VAR_DIR` (undeclared) => `HOST_VAR_DIR`

sources/api/bootstrap-containers/README.md

@@ -44,7 +44,7 @@ USER_DATA_DIR=/.bottlerocket/bootstrap-containers/current
 # This is the in-container view of where the host's `/var` can be accessed.
 HOST_VAR_DIR=/.bottlerocket/rootfs/var
 # The directory that'll be created by this bootstrap container
-MY_HOST_DIR=$VAR_DIR/lib/my_directory
+MY_HOST_DIR=$HOST_VAR_DIR/lib/my_directory
 # Create it!
 mkdir -p "$MY_HOST_DIR"
 # Write the user-data to stdout (to the journal) and to our new path:
@@ -74,4 +74,4 @@ journalctl -u [email protected]
 
 ## Colophon
 
-This text was generated using [cargo-readme](https://crates.io/crates/cargo-readme), and includes the rustdoc from `src/main.rs`.
+This text was generated using [cargo-readme](https://crates.io/crates/cargo-readme), and includes the rustdoc from `src/main.rs`.