Fix bash syntax in sage -upgrade

Trying to run `./sage -upgrade` in 5.9rc0 results in the following:
```
~/Installations/sage-5.9.rc0» ./sage -upgrade
/home/punarbasu/Installations/sage-5.9.rc0/src/bin/sage-upgrade: command substitution: line 30: syntax error near unexpected token `)'
```
This pull request fixes this.