bitbake: bitbake-user-manual: Fixed section head typo

Unseting -> Unsetting

(Bitbake rev: 238a936f60cbffb73f7b370b867e45f91925951c)

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

bitbake/doc/bitbake-user-manual/bitbake-user-manual-metadata.xml

@@ -503,7 +503,7 @@
         </section>
 
         <section id='unsetting-variables'>
-            <title>Unseting variables</title>
+            <title>Unsetting variables</title>
 
             <para>
                 It is possible to completely remove a variable or a variable flag