Compilation error on Linux

[jacobeva]

Each time I try to compile a sketch using this library on Linux I get the following error:

Error during build: fork/exec python /home/user/.arduino15/packages/Heltec_nRF52/hardware/Heltec_nRF52/1.7.0/tools/uf2conv/uf2conv.py: no such file or directory

Despite the directory for the script existing on my system. On further inspection it appeared that one of the recipe targets was strangely written in the platforms.txt. It has been corrected in PR #3 and sketch compilation works again.