chore: post-rename audit cleanup

- Drop the corrupt logo_midi2_cpp.png left over from the bulk sed pass
  (the rename moved logo_midi2_cpp.png to logo_midi2cpp.png; the file
  with the old name should not have come back).
- logo_midi2cpp.png touched up.
- Fix one stale token in README.md Contents (`midi2\_cpp` link text
  pointing at #midi2cpp -> `midi2cpp`).

Author: sauloverissimo

README.md

@@ -27,7 +27,7 @@ Underneath, [midi2](https://github.com/sauloverissimo/midi2) (the portable C99 c
 
 ## Contents
 
-- [midi2\_cpp](#midi2cpp)
+- [midi2cpp](#midi2cpp)
     - [MIDI 2.0 engine for embedded systems](#midi-20-engine-for-embedded-systems)
   - [The library](#the-library)
   - [Contents](#contents)