Adopt wg-rust-by-example as a subteam of lang-docs

We're working to move things out from under the launching-pad team to
more appropriate places.  It makes the most sense for the
rust-by-example working group to be adopted as a subteam of lang-docs,
so let's do that.

teams/wg-rust-by-example.toml

@@ -1,5 +1,5 @@
 name = "wg-rust-by-example"
-subteam-of = "launching-pad"
+subteam-of = "lang-docs"
 kind = "working-group"
 
 [people]