Fix example for "Set Diff Preferences" in REST API documentation

The "Set Diff Preferences" endpoint takes a PUT request, but the
example was using "GET".

Change-Id: I35fc19e820dd46e16e6697ed3b396fec4bb24411

Documentation/rest-api-accounts.txt

diff --git a/Documentation/rest-api-accounts.txt b/Documentation/rest-api-accounts.txt
index 3cdbcb3..0d3ce24 100644
--- a/Documentation/rest-api-accounts.txt
+++ b/Documentation/rest-api-accounts.txt
@@ -1283,7 +1283,7 @@
 
 .Request
 ----
-  GET /a/accounts/self/preferences.diff HTTP/1.0
+  PUT /a/accounts/self/preferences.diff HTTP/1.0
   Content-Type: application/json; charset=UTF-8
 
   {