memcache: fix printf format specifier in AtomCache::load_list()
memcache: fix printf format specifier in AtomCache::load_list()

Found by cppcheck:
[opencog/persist/memcache/AtomCache.cc:229]: (portability) %d in format string
(no. 2) requires 'int' but the argument type is 'size_t {aka unsigned long}'.

rTreutlein/atomspace,AmeBel/atomspace,rTreutlein/atomspace,AmeBel/atomspace,AmeBel/atomspace,AmeBel/atomspace,rTreutlein/atomspace,rTreutlein/atomspace,AmeBel/atomspace,rTreutlein/atomspace