Fix labels for regression issue template

Each label needs to be separated by a comma (see the ICE issue template
for an example of correct usage).

As a result of this problem, the `regression-untriaged` label has not
been automatically added to issues opened with this template.

See c127530 for another example of this.

.github/ISSUE_TEMPLATE/regression.md

@@ -1,7 +1,7 @@
 ---
 name: Regression
 about: Report something that unexpectedly changed between Rust versions.
-labels: C-bug regression-untriaged
+labels: C-bug, regression-untriaged
 ---
 <!--
 Thank you for filing a regression report! 🐛 A regression is something that changed between versions of Rust but was not supposed to.