Update link for prime signs

As it's in the docs, we should link to the docs rather than a GitHub issue.

clicommand/prime-signs.md

@@ -5,7 +5,7 @@ CLI helptext, ie:
 ```
 --disconnect-after-job  Disconnect the agent after running exactly one job. When used in conjunction with the ′--spawn′ flag, each worker booted will run exactly one job [$BUILDKITE_AGENT_DISCONNECT_AFTER_JOB]
 ```
-The quotes around --spawn aren't backticks or quotes, but a [prime symbol](https://en.wikipedia.org/wiki/Prime_(symbol)). The reasoning behind this is that urfave/cli, our CLI library, has [special handling](https://github.com/urfave/cli/issues/333) for backticks (`) in helptext, which in our case leads to some (but not all 🙃) of the backticks getting swallowed.
+The quotes around --spawn aren't backticks or quotes, but a [prime symbol](https://en.wikipedia.org/wiki/Prime_(symbol)). The reasoning behind this is that urfave/cli, our CLI library, has [special handling](https://cli.urfave.org/v1/examples/flags/#placeholder-values) for backticks (`) in helptext, which in our case leads to some (but not all 🙃) of the backticks getting swallowed.
 
 Our workaround for this is to use symbols that look kinda, but not exactly like backticks. Oy.
 ′′′′′′′′′′′′′′′′′′′′′′′′′′′′′′′′′′′′′′′′′′′′