doc/doxygen/chapters/15out_of_core.doxy: fix spelling mistake

doc/doxygen/chapters/15out_of_core.doxy

@@ -43,9 +43,9 @@ Don't forget to check if the result is correct!
 This can also be achieved by just setting environment variables:
 
 \verbatim
-exoprt STARPU_DISK_SWAP=/tmp
-exoprt STARPU_DISK_SWAP_BACKEND=unistd
-exoprt STARPU_DISK_SWAP_SIZE=$((200*1024*1024))
+export STARPU_DISK_SWAP=/tmp
+export STARPU_DISK_SWAP_BACKEND=unistd
+export STARPU_DISK_SWAP_SIZE=$((200*1024*1024))
 \endverbatim
 
 When the register function is called, StarPU will benchmark the disk. This can