documentation/README: correct heading styles actually used in the YP manuals

Sphinx seems to offer flexibility in heading styles definitions,
but the new text matches the way subsubsections and subsubsubsections
are defined in the current manuals.

Note that it's not possible to mix different definitions, for example
by using "~~~~" and "^^^^" for subsubsections in different places.

(From yocto-docs rev: 89a9af58adaa4cf4708d0383655b992f8c7239e2)

Signed-off-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/README

@@ -175,17 +175,18 @@ The layout of the Yocto Project manuals is organized as follows
     Book
       Chapter
         Section
-          Section
-            Section
-
-Here are the heading styles defined in Sphinx:
-
-    Book              => overline ===
-      Chapter         => overline ***
-        Section       => ====
-          Section     => ----
-            Section   => ^^^^
-              Section => """" or ~~~~
+          Subsection
+            Subsubsection
+              Subsubsubsection
+
+Here are the heading styles that we use in the manuals:
+
+    Book                       => overline ===
+      Chapter                  => overline ***
+        Section                => ====
+          Subsection           => ----
+            Subsubsection      => ~~~~
+              Subsubsubsection => ^^^^
 
 With this proposal, we preserve the same TOCs between Sphinx and Docbook.