doxygen: fix correct use of doc/core subdirectory
diff --git a/Doxyfile.in b/Doxyfile.in
index 692c79a..18eb226 100644
--- a/Doxyfile.in
+++ b/Doxyfile.in
@@ -51,7 +51,7 @@
# If a relative path is entered, it will be relative to the location
# where doxygen was started. If left blank the current directory will be used.
-OUTPUT_DIRECTORY = doc
+OUTPUT_DIRECTORY = doc/core
# If the CREATE_SUBDIRS tag is set to YES, then doxygen will create
# 4096 sub-directories (in 2 levels) under the output directory of each output