[README] too broad search term for the Debian and Ubuntu repository badges

The search term for the badge is:
```
https://packages.debian.org/search?keywords=task&searchon=names&suite=all§ion=all
```

And this is headed with a message:
```
Your keyword was too generic, for optimizing reasons some results might have been suppressed.
Please consider using a longer keyword or more keywords.
```

As it catches all the task packages from Debian: https://wiki.debian.org/DebianDesktop/Tasks

Meanwhile the Taskwarrior package in Debian is named “taskwarrior” (surprising, I know ;p), and changing the search term yields the correct results:
```
https://packages.debian.org/search?suite=all§ion=all&arch=any&searchon=names&keywords=taskwarrior
```

I assume that it's the same for the Ubuntu badge, as the search returns only the “too broad” message and searching for “taskwarrior” breaks the search.