[coroutine] Add missing presplitcoroutine attr to example

Signed-off-by: Itay Bookstein <itay.bookstein@nextsilicon.com>

Author: nextsilicon-itay-bookstein

llvm/docs/Coroutines.rst

@@ -285,7 +285,7 @@ The LLVM IR for this coroutine looks like this:
 
 .. code-block:: llvm
 
-  define i8* @f(i32 %n) {
+  define i8* @f(i32 %n) presplitcoroutine {
   entry:
     %id = call token @llvm.coro.id(i32 0, i8* null, i8* null, i8* null)
     %size = call i32 @llvm.coro.size.i32()