rfc10: fix typo

Problem: a capitalziation error was introduced into RFC 10 by
a recent PR.

Fix capitalization.

Author: garlick

spec_10.rst

@@ -158,7 +158,7 @@ non-essential entries from its cache.
 Garbage Collection
 ~~~~~~~~~~~~~~~~~~
 
-References to content are the responsibility of the Flux key Value Store.
+References to content are the responsibility of the Flux Key Value Store.
 Content that the KVS no longer references MAY NOT be removed while the Flux
 instance is running.