qapi: fix documentation example

The QMP wire format uses "", not '', around strings.

* docs/qapi-code-gen.txt: Fix typo.
Signed-off-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

docs/qapi-code-gen.txt

@@ -164,7 +164,7 @@ This example allows using both of the following example objects:
  { "file": "my_existing_block_device_id" }
  { "file": { "driver": "file",
              "readonly": false,
-             'filename': "/tmp/mydisk.qcow2" } }
+             "filename": "/tmp/mydisk.qcow2" } }
 
 
 === Commands ===