README.md: remove mention of the main tree patch

The patch has recently been merged into the main tree.

Signed-off-by: Hugo Lefeuvre <hugo.lefeuvre@manchester.ac.uk>

Author: hlef

README.md

@@ -6,7 +6,6 @@ Unikraft as an external library.
 
 How to use this allocator in your unikernel application:
 
-- apply main-tree.patch to the main tree
 - select "Mimalloc" in `ukboot > Default memory allocator`
 - pass at least 256MiB of memory to the unikernel