fix(samples): correct usage of toString() (#117)

* Remove example's unused character encoding

When accessing a secret version, the payload data is of type `string | Uint8Array`, neither of which accepts a character encoding option in their `toString()` function.

Co-authored-by: Stephen <stephenplusplus@users.noreply.github.com>

secret-manager/accessSecretVersion.js

@@ -34,7 +34,7 @@ async function main(name = 'projects/my-project/secrets/my-secret/versions/1') {
     });
 
     // Extract the payload as a string.
-    const payload = version.payload.data.toString('utf8');
+    const payload = version.payload.data.toString();
 
     // WARNING: Do not print the secret in a production environment - this
     // snippet is showing how to access the secret material.