Update README.md (#401)

added "comma-seperated" the description for `env_vars`

This is important when using the literal block scalar `|` with a
variable `${{<variable name>}}`, as entries are not parsed separately
unless they are comma-seperated (newlines aren't enough)

example:

```
      - id: "deploy"
        uses: "google-github-actions/deploy-cloud-functions@v2"
        with:
          name: "function_name"
          entry_point: "main"
          runtime: "python311"
          memory_mb: "8192MB"
          env_vars: | 
            FOO=${{ env.PROJECT_NAME }}
            BAR=BAZ
          source_dir: ...
```

sets one environment variable `FOO` as `<project_name>\nBAR=BAZ`

Signed-off-by: jehan60188 <21280121+jehan60188@users.noreply.github.com>

Author: jehan60188

README.md

@@ -67,7 +67,7 @@ jobs:
 
 - `region`: (Optional) [Region](https://cloud.google.com/functions/docs/locations) in which the function should be deployed. Defaults to `us-central1`.
 
-- `env_vars`: (Optional) List of key-value pairs to set as environment variables in the format: `KEY1=VALUE1,KEY2=VALUE2`. All existing environment variables will be removed, even if this parameter is not passed. Keys or values that contain a separator must be escaped with a backslash (`\,`, `\\n`). All leading and trailing whitespace is trimmed.
+- `env_vars`: (Optional) List of comma-seperated key-value pairs to set as environment variables in the format: `KEY1=VALUE1,KEY2=VALUE2`. All existing environment variables will be removed, even if this parameter is not passed. Keys or values that contain a separator must be escaped with a backslash (`\,`, `\\n`). All leading and trailing whitespace is trimmed.
 
 - `env_vars_file`: (Optional) Path to a local YAML file with definitions for all environment variables. An example env_vars_file can be found [here](tests/env-var-files/test.good.yaml). All existing environment variables will be removed, even if this parameter is not passed. If `env_vars` is also given, values in `env_vars` take precedence over these values.