fix ethash.go:133: Sprintf format %d reads arg #2, but call has only 1 arg

[mkrufky]

BEFORE applying this fix:

$ make
[...]
Running 21 test cases...

*** No errors detected
################# Testing Go ##################
# _/home/mk/git/ethereum-ethash
./ethash.go:133: Sprintf format %d reads arg #2, but call has only 1 arg
FAIL	_/home/mk/git/ethereum-ethash [build failed]
Makefile:3: recipe for target 'test' failed
make: *** [test] Error 2

AFTER applying this fix:

Running 21 test cases...

*** No errors detected
################# Testing Go ##################
PASS
ok  	_/home/mk/git/ethereum-ethash	2.403s