message: Adds a missing slic directory to the doxygen index
diff: @@ -803,6 +803,7 @@ INPUT = @PROJECT_SOURCE_DIR@/axom/mainpage.md \
  @PROJECT_SOURCE_DIR@/axom/slam/policies \
  @PROJECT_SOURCE_DIR@/axom/slic/README.md \
  @PROJECT_SOURCE_DIR@/axom/slic/core \
  + @PROJECT_SOURCE_DIR@/axom/slic/interface \
  @PROJECT_SOURCE_DIR@/axom/slic/streams \
  @PROJECT_SOURCE_DIR@/axom/spin/README.md \
  @PROJECT_SOURCE_DIR@/axom/spin
  