Small change to Makefile.graham.

Updated the number of threads/blocks used with Graham for better performance.

Makefile.graham

@@ -1,6 +1,6 @@
 # to use, copy this file to 'Makefile.local'
 
-MACROS = -DGRAHAM -DNTHREADS=12800 -DNBLOCKS=20
+MACROS = -DGRAHAM -DNTHREADS=71680 -DNBLOCKS=112
 CUDA_ARCH = sm_60
 
 GMP ?=