doxygen: fix one typo issue (#10200)

Signed-off-by: Chen Wang <unicorn_wang@outlook.com>

documentation/3.kernel/INDEX.md

@@ -1,4 +1,4 @@
-@page page_kernel_core Kenrel
+@page page_kernel_core Kernel
 
 - @subpage page_kernel_basics
 - @subpage page_thread_management