doc: fix link from bootstrap README to BUILDING

PR-URL: #28504
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Yongsheng Zhang <zyszys98@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

Author: rvagg

tools/bootstrap/README.md

@@ -1,2 +1,2 @@
-See the main project [README.md](../../README.md#boxstarter) for details on how
-to use this script.
+See the main project [BUILDING.md](../../BUILDING.md#boxstarter) for details on
+how to use this script.