Update README.md

sjsdfg · web-flow · commit 89a3da5a57ae · 2019-07-17T13:15:13.000+08:00
&#31532;&#21313;&#20843;&#31456; &#23383;&#31526;&#20018;&#26356;&#26032;&#23436;&#25104;
diff --git a/README.md b/README.md
@@ -37,7 +37,7 @@
 - [x] [&#31532;&#21313;&#20116;&#31456; &#24322;&#24120;](docs/book/15-Exceptions.md)
 - [ ] [&#31532;&#21313;&#20845;&#31456; &#20195;&#30721;&#26657;&#39564;](docs/book/16-Validating-Your-Code.md)
 - [ ] [&#31532;&#21313;&#19971;&#31456; &#25991;&#20214;](docs/book/17-Files.md)
-- [ ] [&#31532;&#21313;&#20843;&#31456; &#23383;&#31526;&#20018;](docs/book/18-Strings.md)
+- [x] [&#31532;&#21313;&#20843;&#31456; &#23383;&#31526;&#20018;](docs/book/18-Strings.md)
 - [ ] [&#31532;&#21313;&#20061;&#31456; &#31867;&#22411;&#20449;&#24687;](docs/book/19-Type-Information.md) 
 - [ ] [&#31532;&#20108;&#21313;&#31456; &#27867;&#22411;](docs/book/20-Generics.md)
 - [ ] [&#31532;&#20108;&#21313;&#19968;&#31456; &#25968;&#32452;](docs/book/21-Arrays.md)

-Original file line number
+Diff line change
 - [x] [第十五章 异常](docs/book/15-Exceptions.md)
 - [ ] [第十六章 代码校验](docs/book/16-Validating-Your-Code.md)
 - [ ] [第十七章 文件](docs/book/17-Files.md)
 -- [ ] [第十八章 字符串](docs/book/18-Strings.md)
 +- [x] [第十八章 字符串](docs/book/18-Strings.md)
 - [ ] [第十九章 类型信息](docs/book/19-Type-Information.md)
 - [ ] [第二十章 泛型](docs/book/20-Generics.md)
 - [ ] [第二十一章 数组](docs/book/21-Arrays.md)