Added missing comma in openrc/src/rc
Added missing comma in openrc/src/rc

Clang was failing with:
```
/zroot/jenkins/workspace/update_ghsotbsd-13_poudriere_jail/sbin/openrc/../../contrib/openrc/src/rc/rc.c:70:2: error: suspicious concatenation of string literals in an array initialization; did you mean to separate the elements with a comma? [-Werror,-Wstring-concatenation]
        "when leaving single user or boot runlevels",
        ^
/zroot/jenkins/workspace/update_ghsotbsd-13_poudriere_jail/sbin/openrc/../../contrib/openrc/src/rc/rc.c:69:2: note: place parentheses around the string literal to silence warning
        "override the next runlevel to change into\n"
        ^
```

This fixes #469.

OpenRC/openrc,OpenRC/openrc,williamh/openrc,OpenRC/openrc,williamh/openrc,williamh/openrc