documentation: poky-ref-manual - Fixed grammar typo.

(From yocto-docs rev: 2660f17b79a36772081e37117be85ee304b78f20)

Signed-off-by: Scott Rifenbark <scott.m.rifenbark@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/poky-ref-manual/ref-variables.xml

@@ -669,7 +669,7 @@
                     <literallayout class='monospaced'>
      FILESEXTRAPATHS_prepend := "path_1:path_2:path_3:"
                     </literallayout>
-                    Typically, you want your directories search first. 
+                    Typically, you want your directories searched first. 
                     To make sure that happens, use <filename>_prepend</filename> and 
                     the immediate expansion (<filename>:=</filename>) operator as shown in the 
                     previous example.