doc: Update system.sstables table schema description

The partition key had been renamed and its type changed some time ago,
but the doc wasn't updated. Fix it.

refs: #20998

Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>

Closes scylladb/scylladb#22683

docs/dev/system_keyspace.md

@@ -177,13 +177,13 @@ The "ownership" table for non-local sstables
 Schema:
 ~~~
 CREATE TABLE system.sstables (
-    location text,
+    owner uuid,
     generation timeuuid,
     format text,
     status text,
     uuid uuid,
     version text,
-    PRIMARY KEY (location, generation)
+    PRIMARY KEY (owner, generation)
 )
 ~~~