Reflect update to Android NDK r17c in the documentation
### `default`
* `android`
* `i686`, `x86_64`, `armv7a`, `armv7a_neon`, `armv8a`
- Clang 6.0.2, Android NDK r17b. API 14 for the 32-bit platforms, API 21 for 64-bit.
+ Clang 6.0.2, Android NDK r17c. API 14 for the 32-bit platforms, API 21 for 64-bit.
* `darwin`
* `x86_64`
Clang 7.0.0, macOS SDK 10.13
