Typo

Updated the English README to fix some minor grammatical and spelling mistakes

Please respect the specification of C

make %(foo) work

Fix for Linux

Text corrections.

the type of return values of tr->func_ii() and tr->func_si() is already mrb_value, so no need to cast.

MRUBY_OBJECT_HEADER is always used with `;', but, in the definition of a struct, you cannot include any statement except declarations, even if the statement is void statement.

In mruby.h, there is no definitions of RSTRING_* here, so cannot compile if the compiler is not GCC.