docs: Fix a typo in the GVariant text format documentation
docs: Fix a typo in the GVariant text format documentation

There were some quotes missing from an a{sv} key.

Signed-off-by: Philip Withnall <29c2a2ac2a854b9cf047275aa9de346d096be2e0@endlessm.com>

endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib