simple naming mistake

this must be a different endpoint or it is ambiguous with bucket
interactions.  plus it makes more sense that "place where storage
indexes are" is different from "place where buckets are" although I
am still uncomfortable with the idea that "storage indexes" are things
and not ... indexes ...

docs/proposed/http-storage-node-protocol.rst

@@ -138,7 +138,7 @@ Shares are immutable data stored in buckets.
 Writing
 ~~~~~~~
 
-``POST /v1/buckets/:storage_index``
+``POST /v1/storage/:storage_index``
 !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
 
 Create some new buckets in which to store some shares.