typo: `ignwored` at line 77, `friend` at line 89, `prsevious` at line 401

[wangyu-ustc]

fixes #161

• fix extra letter typo ignwored at line 77 to ignored @wangyu-ustc
• fix skip letter typo friend at line 89 to friends @wangyu-ustc
• fix wrong letter typo prsevious at line 401 to previous @10205501431

team: 10205501431 10205501418

[TieWay59]

嗨，我想我们作业指南里面写的尽量不要创建新PR的方式重做。不过我也不在这方面特别为难，fixes 后面填写的issue号应该是 #139 不是修复这个PR。