Minor typo: \subsubsections --> \subsubsection

Doc/doc/doc.tex

@@ -364,7 +364,7 @@ This \UNIX\ is also followed by a space.
         \lineiii{1}{\macro{chapter}}{(1)}
         \lineiii{2}{\macro{section}}{}
         \lineiii{3}{\macro{subsection}}{}
-        \lineiii{4}{\macro{subsubsections}}{}
+        \lineiii{4}{\macro{subsubsection}}{}
         \lineiii{5}{\macro{paragraph}}{(2)}
         \lineiii{6}{\macro{subparagraph}}{}
       \end{tableiii}