Fix bug

Automatic download of dependencies is considered to be the default
behavior when building the project according to README.md.

However, the automatic download is not enabled by default now due to
the mistake in syntax of option() command in CMakeLists.txt.

This commit fixes the bug.

CMakeLists.txt

 cmake_minimum_required(VERSION 3.14)
 project(exple_cmake_build CXX)
 
-option(DOWNLOAD_DEPS ON)
+option(DOWNLOAD_DEPS "Enable automatic download of dependencies" ON)
 
 # download dependency 'explelibx_common' or find it locally
 if (DOWNLOAD_DEPS)