Fix boost build standalone documentation.

Boost build defines its own jamroot, so it wasn't picking up the
`boost.defaults` flag from the main jamroot.

[SVN r68649]

tools/build

@@ -1 +1 @@
-Subproject commit d8678f9bf764774f03a7e119854cd1cb0a22ac23
+Subproject commit 48534170352fac0a97c5048e24b903f80f067439