Updated git clone line in install instructions (#367)
Updated git clone line in install instructions (#367)

Summary:
Get the following when I run `git clone git@github.com:facebookresearch/detectron2.git`. Replaced line with `git clone https://github.com/facebookresearch/detectron2.git`.

```bash
git clone git@github.com:facebookresearch/detectron2.git
Cloning into 'detectron2'...
The authenticity of host 'github.com (192.30.253.113)' can't be established.
RSA key fingerprint is SHA256:nThbg6kXUpJWGl7E1IGOCspRomTxdCARLviKw6E5SY8.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added 'github.com,192.30.253.113' (RSA) to the list of known hosts.
git@github.com: Permission denied (publickey).
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.
```
Pull Request resolved: https://github.com/facebookresearch/detectron2/pull/367

Differential Revision: D18646068

Pulled By: ppwwyyxx

fbshipit-source-id: 3254dda0a6386cfa3a17fdda0e875f6badeee26a

facebookresearch/detectron2,facebookresearch/detectron2,facebookresearch/detectron2