Update docs/ref/spec.md

Co-authored-by: Ryan Bottriell <[email protected]>

Author: dcookspi

docs/ref/spec.md

@@ -126,7 +126,7 @@ options to each build, as described in the table:
 
 |  Value     | Adds these host options                        |
 | ---------- | ---------------------------------------------- |
-| **Distro** | "distro", "arch", "os", and the "<distroname>" |
+| **Distro** (default) | "distro", "arch", "os", and the "<distroname>" |
 | **Arch**   | "arch", "os", and the "<distroname>"           |
 | **Os**     | "os"                                           |
 | **None**   |                                                |