[AIRFLOW-2503] Fix broken links in CONTRIBUTING.md
[AIRFLOW-2503] Fix broken links in CONTRIBUTING.md

- Fix broken links in `CONTRIBUTING.md`

Closes #3397 from kaxil/AIRFLOW-2503

artwr/airflow,wooga/airflow,mrkm4ntr/incubator-airflow,danielvdende/incubator-airflow,CloverHealth/airflow,apache/airflow,mistercrunch/airflow,sid88in/incubator-airflow,mtagle/airflow,cfei18/incubator-airflow,adamhaney/airflow,Acehaidrey/incubator-airflow,lyft/incubator-airflow,mistercrunch/airflow,nathanielvarona/airflow,subodhchhabra/airflow,fenglu-g/incubator-airflow,sid88in/incubator-airflow,bolkedebruin/airflow,wooga/airflow,sekikn/incubator-airflow,nathanielvarona/airflow,apache/incubator-airflow,nathanielvarona/airflow,akosel/incubator-airflow,nathanielvarona/airflow,malmiron/incubator-airflow,apache/incubator-airflow,danielvdende/incubator-airflow,wooga/airflow,wooga/airflow,airbnb/airflow,subodhchhabra/airflow,DinoCow/airflow,gtoonstra/airflow,danielvdende/incubator-airflow,dhuang/incubator-airflow,fenglu-g/incubator-airflow,spektom/incubator-airflow,malmiron/incubator-airflow,DinoCow/airflow,artwr/airflow,Acehaidrey/incubator-airflow,malmiron/incubator-airflow,criccomini/airflow,apache/airflow,owlabs/incubator-airflow,r39132/airflow,cfei18/incubator-airflow,wileeam/airflow,gtoonstra/airflow,jgao54/airflow,lxneng/incubator-airflow,dhuang/incubator-airflow,DinoCow/airflow,jgao54/airflow,danielvdende/incubator-airflow,spektom/incubator-airflow,akosel/incubator-airflow,lxneng/incubator-airflow,airbnb/airflow,criccomini/airflow,dhuang/incubator-airflow,malmiron/incubator-airflow,wileeam/airflow,mtagle/airflow,subodhchhabra/airflow,wileeam/airflow,andyxhadji/incubator-airflow,lyft/incubator-airflow,airbnb/airflow,mrkm4ntr/incubator-airflow,subodhchhabra/airflow,lxneng/incubator-airflow,apache/airflow,yk5/incubator-airflow,mrkm4ntr/incubator-airflow,wolfier/incubator-airflow,owlabs/incubator-airflow,artwr/airflow,adamhaney/airflow,sergiohgz/incubator-airflow,andyxhadji/incubator-airflow,mistercrunch/airflow,fenglu-g/incubator-airflow,DinoCow/airflow,edgarRd/incubator-airflow,mistercrunch/airflow,yk5/incubator-airflow,Acehaidrey/incubator-airflow,danielvdende/incubator-airflow,CloverHealth/airflow,bolkedebruin/airflow,wolfier/incubator-airflow,Fokko/incubator-airflow,owlabs/incubator-airflow,yk5/incubator-airflow,lxneng/incubator-airflow,adamhaney/airflow,andyxhadji/incubator-airflow,cfei18/incubator-airflow,lyft/incubator-airflow,spektom/incubator-airflow,r39132/airflow,jgao54/airflow,r39132/airflow,nathanielvarona/airflow,sid88in/incubator-airflow,Acehaidrey/incubator-airflow,artwr/airflow,mtagle/airflow,sergiohgz/incubator-airflow,CloverHealth/airflow,airbnb/airflow,apache/airflow,wolfier/incubator-airflow,CloverHealth/airflow,sergiohgz/incubator-airflow,yk5/incubator-airflow,cfei18/incubator-airflow,edgarRd/incubator-airflow,sergiohgz/incubator-airflow,gtoonstra/airflow,dhuang/incubator-airflow,owlabs/incubator-airflow,wolfier/incubator-airflow,apache/airflow,akosel/incubator-airflow,Fokko/incubator-airflow,Acehaidrey/incubator-airflow,cfei18/incubator-airflow,andyxhadji/incubator-airflow,mtagle/airflow,Fokko/incubator-airflow,edgarRd/incubator-airflow,spektom/incubator-airflow,bolkedebruin/airflow,gtoonstra/airflow,lyft/incubator-airflow,mrkm4ntr/incubator-airflow,sekikn/incubator-airflow,r39132/airflow,apache/airflow,sekikn/incubator-airflow,wileeam/airflow,sid88in/incubator-airflow,sekikn/incubator-airflow,fenglu-g/incubator-airflow,edgarRd/incubator-airflow,apache/incubator-airflow,bolkedebruin/airflow,criccomini/airflow,criccomini/airflow,danielvdende/incubator-airflow,nathanielvarona/airflow,Acehaidrey/incubator-airflow,akosel/incubator-airflow,Fokko/incubator-airflow,apache/incubator-airflow,jgao54/airflow,bolkedebruin/airflow,cfei18/incubator-airflow,adamhaney/airflow