windows_install_prerequisites: Fix generation of PATH command

Accidental regression in 7c8d5d2 - escape
character didn't move with PATH.

Reported on forum https://esp32.com/viewtopic.php?f=13&t=8201

tools/windows/windows_install_prerequisites.sh

@@ -54,7 +54,7 @@ rm ~/${TOOLCHAIN_ZIP}
 cat > /etc/profile.d/esp32_toolchain.sh << EOF
 # This file was created by ESP-IDF windows_install_prerequisites.sh
 # and will be overwritten if that script is run again.
-export PATH="\/opt/xtensa-esp32-elf/bin:$PATH"
+export PATH="/opt/xtensa-esp32-elf/bin:\$PATH"
 EOF
 
 # clean up pacman package cache to save some disk space