Android: Update min required NDK version

It needs to have the now recommended clang toolchain,
r10e is at least no longer supported since this commit [0],
I bumped to a known working version (could be that r12 works too).

[0] https://github.com/godotengine/godot/commit/b18ff942be0dd89bda32e01cc30a41c196fc9290
(cherry picked from commit 09b78d075f0f6ae4ab779246ef3faa8f3e5bc6a1)

reference/compiling_for_android.rst

@@ -26,7 +26,7 @@ For compiling under Windows, Linux or OSX, the following is required:
 -  [Windows only] PyWin32 (optional, for parallel compilation)
 -  Android SDK version 23.0.3 [Note: Please install all Tools and Extras of sdk manager]
 -  Android build tools version 19.1
--  Android NDK r10e or later
+-  Android NDK r13 or later
 -  Gradle (will be downloaded and installed automatically if missing)
 -  JDK 6 or later (either OpenJDK or Oracle JDK) - JDK 9 is untested as of now