fix MappedLun typo
fix MappedLun typo

Introduced in c37a00b93bff4d89b28992e7c4e6d1fa98351b74,
which is in fb38 and fb39.

The reported error was
NameError: global name 'MappedLun' is not defined.

Signed-off-by: Johannes Dewender <64b2b6d12bfe4baae7dad3d018f8cbf6b0e7a044@JonnyJD.net>

cvubrugier/rtslib-fb,mikenawrocki/rtslib-fb,agrover/rtslib-fb,mikenawrocki/rtslib-fb