snapshots doc: Repository Verification `verify` example fix

Replace `my_backup` in the example line by `s3_repository` to match
the example above.

docs/reference/modules/snapshots.asciidoc

@@ -186,7 +186,7 @@ The verification process can also be executed manually by running the following
 
 [source,js]
 -----------------------------------
-POST /_snapshot/my_backup/_verify
+POST /_snapshot/s3_repository/_verify
 -----------------------------------
 // AUTOSENSE