doc: fix a typo
windows: fix conflicted declaration in MinGW environment

This change fixes following error:

  /usr/lib/gcc/i686-w64-mingw32/4.6/../../../../i686-w64-mingw32/include/stdio.h:339:5:
  error: previous declaration of 'int _snprintf(char*, size_t, const
  char*, ...)' with 'C++' linkage
  /usr/lib/gcc/i686-w64-mingw32/4.6/../../../../i686-w64-mingw32/include/stdio.h:532:110:
  error: conflicts with new declaration with 'C' linkage
windows: fix conflicted declaration in MinGW enrironment

This change fixes following error:

  /usr/lib/gcc/i686-w64-mingw32/4.6/../../../../i686-w64-mingw32/include/stdio.h:350:5:
  error: previous declaration of 'int _vsnprintf(char*, size_t, const
  char*, char*)' with 'C++' linkage
  /usr/lib/gcc/i686-w64-mingw32/4.6/../../../../i686-w64-mingw32/include/stdio.h:534:121:
  error: conflicts with new declaration with 'C' linkage