Update README.md

Update image URLs to make the validation step happy.

README.md

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 ### Supported variables
 The plugin currently supports variables for logging scopes. For example, you can define a project variable and switch between projects. The following screenshot shows an example using project, bucket, and view.
 
-![template variables](./src/img/template_vars.png?raw=true)
+![template variables](https://github.com/GoogleCloudPlatform/cloud-logging-data-source-plugin/blob/main/src/img/template_vars.png?raw=true)
 
 Below is an example of defining a variable for log views.
-![define a variable](./src/img/template_query_vars.png?raw=true)
+![define a variable](https://github.com/GoogleCloudPlatform/cloud-logging-data-source-plugin/blob/main/src/img/template_query_vars.png?raw=true)
 ## Licenses
 
 Cloud Logging Logo (`src/img/logo.svg`) is from Google Cloud's [Official icons and sample diagrams](https://cloud.google.com/icons)