fix Android build command in README

Using `*-linux-android29-clang` results in errors, adding `++` fixes this.

Credits go to zhangshiqian1214:
https://github.com/godotengine/godot-cpp/issues/372#issuecomment-596079055

README.md

@@ -256,11 +256,11 @@ submit a pull request :slightly_smiling_face:
 
 ```bash
 cd SimpleLibrary
-aarch64-linux-android29-clang -fPIC -o src/init.os -c src/init.cpp -g -O3 -std=c++14 -Igodot-cpp/include -Igodot-cpp/include/core -Igodot-cpp/include/gen -Igodot-cpp/godot_headers
-aarch64-linux-android29-clang -o bin/libtest.so -shared src/init.os -Lgodot-cpp/bin -l<name of the godot-cpp>
+aarch64-linux-android29-clang++ -fPIC -o src/init.os -c src/init.cpp -g -O3 -std=c++14 -Igodot-cpp/include -Igodot-cpp/include/core -Igodot-cpp/include/gen -Igodot-cpp/godot_headers
+aarch64-linux-android29-clang++ -o bin/libtest.so -shared src/init.os -Lgodot-cpp/bin -l<name of the godot-cpp>
 ```
 
-You'll need to replace `<name of the godot-cpp>` with the file that was created in [**Compiling the cpp bindings library**](#compiling-the-cpp-bindings-library). The command above targets `arm64v8`. To target `armv7`, use `armv7a-linux-androideabi29-clang` instead of `aarch64-linux-android29-clang`.
+You'll need to replace `<name of the godot-cpp>` with the file that was created in [**Compiling the cpp bindings library**](#compiling-the-cpp-bindings-library). The command above targets `arm64v8`. To target `armv7`, use `armv7a-linux-androideabi29-clang++` instead of `aarch64-linux-android29-clang++`.
 
 This creates the file `libtest.so` in your `SimpleLibrary/bin` directory.