[roll] Roll fuchsia [realm_builder] Correct doc comment

`get_decl` does not return a mutable reference; change the doc comment
to reflect that.

Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/551462
Original-Revision: 929f5abae6df624c32e98b74bb20a7adfdb6b779
GitOrigin-RevId: 704122014982743f61c08133e794aed45eaaea6c
Change-Id: I27de6edcd057990d88950ccf566f1a5e6c759302
diff --git a/stem b/stem
index 80eb4f0..be36756 100644
--- a/stem
+++ b/stem
@@ -10,7 +10,7 @@
              githooks=""
              remote="https://fuchsia.googlesource.com/fuchsia"
              gerrithost="https://fuchsia-review.googlesource.com"
-             revision="d22a21a5ce9c49e744603a012a070296fa6885df"/>
+             revision="929f5abae6df624c32e98b74bb20a7adfdb6b779"/>
   </projects>
   <hooks>
     <hook name="install-environment"