Typos: replace two occurrences of [#endef] with [#enddef].

README.md

@@ -56,7 +56,7 @@ at the beginning of a line, possibly preceded by some whitespace, and followed
 by a valid directive name or by a number:
 
 ```ocaml
-BLANK* "#" BLANK* ("def"|"endef"|"define"|"undef"
+BLANK* "#" BLANK* ("def"|"enddef"|"define"|"undef"
                   |"if"|"ifdef"|"ifndef"|"else"|"elif"|"endif"
                   |"include"
                   |"warning"|"error"

src/cppo_lexer.mll

@@ -239,7 +239,7 @@ and directive e = parse
   (* #enddef ends a definition, which (we expect) has been opened by #def.
      Because we use the same pair of tokens, namely [DEF] and [ENDEF], for
      both kinds of definitions (#define and #def), it is in fact possible to
-     begin a definition with #define and end it with #endef. We do not
+     begin a definition with #define and end it with #enddef. We do not
      document this fact, and users should not rely on it. *)
   | blank* "enddef"
       { blank_until_eol e lexbuf;