Don't mention Lenna.png in docs

[RollingStar]

https://en.wikipedia.org/wiki/Lenna#Criticism

Lenna comes from a Playboy centerfold. It doesn't seem appropriate to use as a general-purpose example.

[0x09]

@RollingStar the example now uses a reduced size version of the public domain Blue Marble 2012 which is also actually better for demonstrating resdet as it turns out. https://github.com/0x09/resdet/commit/7dfef1059ee2e456a23fc5d79262ca75371aef4c

dspfun has been updated as well as the newer subprojects there had already adopted a different, CC0 example image, so this is now used more consistently: https://github.com/0x09/dspfun/commit/e34b65c2015d38a613837928bd7cbd9b4c61a87c https://github.com/0x09/dspfun/commit/d77fb9e480ad811610837f71741da5a36b3a98b7

[RollingStar]

Thank you!