fix typo in error checking code in MapBlock

Fixes what I appears to be a typo (& instead of % for determining divisibility) and clarifies operator precedence.

gcc-4.8.3 helped find this with the warning:
src/Ruleset/MapBlock.cpp:52:14: warning: & has lower precedence than !=; != will be evaluated first [-Wparentheses]
 if (_size_x & 10 != 0 || _size_y % 10 != 0)
             ^~~~~~~~~
src/Ruleset/MapBlock.cpp:52:14: note: place parentheses around the '!=' expression to silence this warning
 if (_size_x & 10 != 0 || _size_y % 10 != 0)
             ^
               (      )
src/Ruleset/MapBlock.cpp:52:14: note: place parentheses around the & expression to evaluate it first
 if (_size_x & 10 != 0 || _size_y % 10 != 0)
             ^
     (           )