[array.std::tuple_size] Fix examples compilation error.

Examples used Using Directive 'using namespace sprout' and a variable
named 'size'.
This caused a compilation error because 'size' was ambiguous.
('sprout::size' and variable 'size')
So, the variable renamed to 'n'.
Merge pull request #48 from kariya-mitsuru/std-tuple_size-doc

[array.std::tuple_size] Fix examples compilation error.