ref-manual: Fixed a typo.

(From yocto-docs rev: 496d4b9bc1e5cf7f59fbd97ed14257a64c0b560e)

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

documentation/ref-manual/ref-structure.xml

@@ -779,7 +779,7 @@
             This directory contains common license files and several text files
             used by the build system.
             The text files contain minimal device information and
-            lists of files and directories with knows permissions.
+            lists of files and directories with known permissions.
         </para>
     </section>