doc: update CODEOWNER for rst

Add Nanlin (@NanlinXie) as a reviewer for ACRN documentation.

Tracked-On: #3419
Signed-off-by: lirui34 <ruix.li@intel.com>

Author: lirui34

CODEOWNERS

@@ -17,4 +17,4 @@
 /doc/                                    @dbkinder @deb-intel @NanlinXie
 /misc/                                   @terryzouhao @szhen11
 
-*.rst                                    @dbkinder @deb-intel
+*.rst                                    @dbkinder @deb-intel @NanlinXie