pw_kvs: Rename sector_descriptor.h to sectors.h

Preparing for SectorDescriptor list refactor.

Change-Id: I1265bf3986c997e14e92af075853cbb5a8b0f367
diff --git a/pw_kvs/BUILD b/pw_kvs/BUILD
index 76bef4f..f469cce 100644
--- a/pw_kvs/BUILD
+++ b/pw_kvs/BUILD
@@ -36,7 +36,7 @@
         "public/pw_kvs/internal/entry_cache.h",
         "public/pw_kvs/internal/hash.h",
         "public/pw_kvs/internal/key_descriptor.h",
-        "public/pw_kvs/internal/sector_descriptor.h",
+        "public/pw_kvs/internal/sectors.h",
         "public/pw_kvs/internal/span_traits.h",
         "pw_kvs_private/macros.h",
     ],