Docs: Fix expression syntax docs url (#3208)

docs/docs/20-usage/20-workflow-syntax.md

@@ -410,7 +410,7 @@ Passing a defined ignore-message like `[ALL]` inside the commit message will ign
 
 Execute a step only if the provided evaluate expression is equal to true. Both built-in [`CI_`](./50-environment.md#built-in-environment-variables) and custom variables can be used inside the expression.
 
-The expression syntax can be found in [the docs](https://github.com/expr-lang/expr/blob/master/docs/Language-Definition.md) of the underlying library.
+The expression syntax can be found in [the docs](https://github.com/expr-lang/expr/blob/master/docs/language-definition.md) of the underlying library.
 
 Run on pushes to the default branch for the repository `owner/repo`: