fixed first export PATH line in Linux.md (tutorial lesson1 for Linux)

PATH was written using lower case (export path=$path:... instead of export PATH=$PATH:...). The path added pointed to the non-prefixed binaries arm-none-eabi binaries, not the prefixed versions the Makefiles in the examples seem to expect. (e.g: non-prefixed 'gcc' vs 'arm-none-eabi-gcc')