Update chapter_03.rst

删除掉未使用的clcf变量避免编译时出现-Werror=unused-variable错误
Update chapter_02.rst

'0' 修改'\0' 在sphinx中\为反转义符号,所以在普通文字中写的时候需要'\\0'来表示。
Merge pull request #112 from chengpohi/patch-2

Update chapter_03.rst
Merge pull request #113 from chengpohi/patch-3

Update chapter_02.rst
Update chapter_02.rst

修改了ngx_string、ngx_str_null、ngx_str_set、ngx_str_null的说明
Merge pull request #115 from notishell/master

Modify chapter_02.rst, update the introduction of ngx_str_t apis, thanks to notishell.