esp32-blink: rename LED to Led

[kassane]

Fix:

-- Configuring done (2.1s)
CMake Error at main/CMakeLists.txt:63 (target_sources):
  Cannot find source file:

    LED.swift

  Tried extensions .c .C .c++ .cc .cpp .cxx .cu .mpp .m .M .mm .ixx .cppm
  .ccm .cxxm .c++m .h .hh .h++ .hm .hpp .hxx .in .txx .f .F .for .f77 .f90
  .f95 .f03 .hip .ispc


-- Generating done (0.4s)
CMake Generate step failed.  Build files cannot be regenerated correctly.
FAILED: build.ninja 

Tested on ArchLinux (x86_64/zen3) - GPIO i18 (blink-led)

[rauhul]

Thanks for the fix!