Atom feed has incorrect markup for entry links

I noticed that my feed reader isn't able to link to each item. I suspect it may be because the atom feed doesn’t use the correct markup for entry links.

Current markup:
```xml
 <link>https://jobs.code4lib.org/jobs/42488-research-information-scientist</link>
```

Correct markup:
```xml
 <link href="https://jobs.code4lib.org/jobs/42488-research-information-scientist" />
```