bug report template: suggest --stacktrace instead of -s (#10548)

1137b183 removed the -s option from spack such that to see a stack
trace you must pass the --stacktrace option. The bug report templates
were not updated accordingly.

.github/ISSUE_TEMPLATE/bug_report.md

@@ -24,10 +24,10 @@ If Spack reported an error, provide the error message. If it did not report an e
 but the output appears incorrect, provide the incorrect output. If there was no error
 message and no output but the result is incorrect, describe how it does not match
 what you expect. To provide more information you might re-run the commands with 
-the additional -sd flags:
+the additional -d/--stacktrace flags:
 ```console
-$ spack -sd <command1> <spec>
-$ spack -sd <command2> <spec>
+$ spack -d --stacktrace <command1> <spec>
+$ spack -d --stacktrace <command2> <spec>
 ...
 ```
 that activate the full debug output. 
@@ -46,4 +46,4 @@ We encourage you to try, as much as possible, to reduce your problem to the mini
 
 If you want to ask a question about the tool (how to use it, what it can currently do, etc.), try the `#general` channel on our Slack first. We have a welcoming community and chances are you'll get your reply faster and without opening an issue.
 
-Other than that, thanks for taking the time to contribute to Spack!
\ No newline at end of file
+Other than that, thanks for taking the time to contribute to Spack!