Update dependency reference from revision to tag in README

Replaced the 'rev' field with 'tag' in the Poetry dependency configuration for better clarity and version management. This ensures more explicit referencing for the specific version.

README.md

@@ -16,7 +16,7 @@ poetry:
 
 ```toml
 [tool.poetry.dependencies]
-youtrack = { git = "https://github.com/hyperskill/hyperskill-sdk.git", rev = "1.0.0", subdirectory = "youtrack" }
+youtrack = { git = "https://github.com/hyperskill/hyperskill-sdk.git", tag = "1.0.0", subdirectory = "youtrack" }
 ```
 
 ```bash