pw_software_update: Fix return type for PrepareForUpdate rpc

Fix typo, return PrepareUpdateResult rather than OperationResult
in PrepareForUpdate.

No-Docs-Update-Reason: Typo fix in proto.
Change-Id: Id5946c27b6a17121c8bffcaa1be7530b61f8ad53
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/61344
Reviewed-by: Eric Hsu <ericchhsu@google.com>
Reviewed-by: Ewout van Bekkum <ewout@google.com>
Commit-Queue: David Rogers <davidrogers@google.com>
diff --git a/pw_software_update/service.proto b/pw_software_update/service.proto
index b8ad924..3765fc0 100644
--- a/pw_software_update/service.proto
+++ b/pw_software_update/service.proto
@@ -81,7 +81,7 @@
   //
   // Device UpdateState should be INACTIVE when calling, will otherwise be
   // rejected.
-  rpc PrepareForUpdate(pw.protobuf.Empty) returns (OperationResult) {};
+  rpc PrepareForUpdate(pw.protobuf.Empty) returns (PrepareUpdateResult) {};
 
   // Verify the bundle that has been transferred to the staging area. Close the
   // pw_transfer channel used for staging bundle.