Adopt the book team as a subteam of lang-docs (#1636)

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

teams/book.toml

@@ -1,5 +1,5 @@
 name = "book"
-subteam-of = "launching-pad"
+subteam-of = "lang-docs"
 
 [people]
 leads = ["carols10cents", "chriskrycho"]