sdk-manual: Replaced "configure.in" with "configure.ac"

The Autotools example called for a "configure.in" file as part
of the "hello-world" example.  When working through the example,
a warning occurs saying that the file needs to be "configure.ac".
I changed the example.

(From yocto-docs rev: f2ec3e564f828ad79f778d47862b4b422ad5a147)

Signed-off-by: Scott Rifenbark <srifenbark@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/sdk-manual/sdk-working-projects.xml

@@ -69,7 +69,7 @@
                                 </literallayout>
                                 </para></listitem>
                             <listitem><para>
-                                For <filename>configure.in</filename>,
+                                For <filename>configure.ac</filename>,
                                 include these lines:
                                 <literallayout class='monospaced'>
      AC_INIT(hello,0.1)