README: Fix typo in *difficult* (sonic-net#192)

Author: paulmenzel

README.md

@@ -9,7 +9,7 @@ This repository contains the scripts and patches to build the kernel for SONiC.
 - Existing kernel modules need to be patched and those patches are available in upstream.
   - Please include the original upstream commit id and message in the patch. This allows the maintainer to remove upstream patches during the kernel upgrade.
 - New kernel modules which are common to all platforms.
-- Platform specific kernel modules which are impossible or very difficul to be built out of kernel tree.
+- Platform specific kernel modules which are impossible or very difficult to be built out of kernel tree.
 
 Platform specific kernel modules are expected to develop out-of-tree kernel modules, provide them in debian packages to be embeded into SONiC ONE image and installed on their platforms.