Fix link to list of supported targets (#327)

This points the "supported-targets" link in the readme back to the
`supported-targets` file, which it had originally linked to and
which its text suggests it is still intended to link to.

After #275, the readme link to the supported targets list pointed
to the `trigger-package-build.py` script. This appears to have been
based on idea of removing the `supported-targets` file. As noted in
https://github.com/cargo-bins/cargo-quickinstall/pull/275/files#r1747798959,
the file was not removed. But the link target was not restored.

As long as the file does exist, it is easier for users reading the
readme to consult it rather than examine a script. In addition, the
link was fully broken since #300 when the script was rewritten and
named `trigger_package_build.py` (with underscores).

README.md

@@ -45,7 +45,7 @@ Edit the command however you need, and paste it into your CI pipeline.
 
 ## Supported targets
 
-Check [supported-targets](/cronjob_scripts/trigger-package-build.py) for lists of targets quickinstall
+Check [supported-targets](/supported-targets) for lists of targets quickinstall
 can build for.
 
 ## Limitations