Documentation: variable substitution in url templates

[Gaeldrin]

When using version substitution in url-template, it counts underscore as part of the variable name (which is ok) without any obvious way to make the underscore part of URL instead (which is not ok as it is impossible to construct real world URL), e.g.:

url: https://get.geo.opera.com/pub/opera/desktop/94.0.4606.38/linux/Opera_94.0.4606.38_amd64.deb
x-checker-data:
  type: anitya
  project-id: 7242
  url-template: https://get.geo.opera.com/pub/opera/desktop/$version/linux/Opera_$version_amd64.deb

results in:

ERROR src.manifest: Failed to check file opera/opera.deb with AnityaChecker: Error substituting template: 'version_amd64'

Edit: I could actually make it work by using folded brackets (e.g. Opera_${version}_amd64.deb) thanks to previous experience with shell but I still think that it could be improvement to mention this in the documentation (together with info if version is the only variable).

[wjt]

https://github.com/flathub/flatpak-external-data-checker#url-templates says:

If the placeholder is immediately followed by an underscore, you need to add braces:

and gives an example. Please open a pull request if you can think of a way to reword the documentation to make this clearer!