[Bug #1170422] Use correct option name

Doc/dist/dist.tex

@@ -403,7 +403,7 @@ any compile/link requirements (include directories, libraries to link
 with, etc.).
 
 All of this is done through another keyword argument to
-\function{setup()}, the \option{extensions} option.  \option{extensions}
+\function{setup()}, the \option{ext_modules} option.  \option{ext_modules}
 is just a list of \class{Extension} instances, each of which describes a
 single extension module.  Suppose your distribution includes a single
 extension, called \module{foo} and implemented by \file{foo.c}.  If no