Help: Fix QNX cross-compile example toolchain file

`q++` is the correct name for C++ compiler.

QNX does not support ARM32 anymore.  Use `aarch64` instead.

Help/manual/cmake-toolchains.7.rst

@@ -234,11 +234,11 @@ value to those supported compilers when compiling:
 
   set(CMAKE_SYSTEM_NAME QNX)
 
-  set(arch gcc_ntoarmv7le)
+  set(arch gcc_ntoaarch64)
 
   set(CMAKE_C_COMPILER qcc)
   set(CMAKE_C_COMPILER_TARGET ${arch})
-  set(CMAKE_CXX_COMPILER QCC)
+  set(CMAKE_CXX_COMPILER q++)
   set(CMAKE_CXX_COMPILER_TARGET ${arch})
 
   set(CMAKE_SYSROOT $ENV{QNX_TARGET})