README: update reference to *.README

Update some reference in README file detected by
scripts/documentation-file-ref-check

*.README => README.*
Signed-off-by: NPatrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: NHeinrich Schuchardt <xypron.glpk@gmx.de>

README

@@ -4639,7 +4639,7 @@ int main(int argc, char *argv[])
 	while (learning) {
 		Read the README file in the top level directory;
 		Read http://www.denx.de/twiki/bin/view/DULG/Manual;
-		Read applicable doc/*.README;
+		Read applicable doc/README.*;
 		Read the source, Luke;
 		/* find . -name "*.[chS]" | xargs grep -i <keyword> */
 	}