artifact -> build artifact

Author: jyn514

src/building/bootstrapping.md

@@ -130,7 +130,7 @@ This is a detailed look into the separate bootstrap stages.
 
 The convention `x.py` uses is that:
 - A `--stage N` flag means to run the stage N compiler (`stageN/rustc`).
-- A "stage N artifact" is an artifact that is _produced_ by the stage N compiler.
+- A "stage N artifact" is a build artifact that is _produced_ by the stage N compiler.
 - The "stage (N+1) compiler" is assembled from "stage N artifacts". This
   process is called _uplifting_.