message: Update plumbing.md
  Fixed a small typo.
diff: @@ -256,7 +256,7 @@ enum HelloWorldGroupKey__ {
  The `Query` trait that we saw earlier includes a method `group_key` for wrapping
  the key for some individual query into the group key.
  -### Group storege
  +### Group storage
  The "group storage" is the actual struct that contains all the hashtables and
  so forth for each query. The types of these are ultimately defined by the
  