vars section fix type and wrap expression syntax in backticks (#35)

Co-authored-by: Casey Lee <[email protected]>

src/usage/index.md

@@ -108,7 +108,7 @@ Example:
 
 # Vars
 
-To run `act` with repository variables that are accessible inside the workflow via ${{ vars.VARIABLE }}, you can enter them interactively or load them from a file. The following options are available for providing github repository variables:
+To run `act` with repository variables that are accessible inside the workflow via `${{ vars.VARIABLE }}`, you can enter them interactively or load them from a file. The following options are available for providing github repository variables:
 
 - `act --var VARIABLE=somevalue` - use `somevalue` as the value for `VARIABLE`.
 - `act --var-file my.variables` - load variables values from `my.variables` file.