[extension/oauth2clientauth] fix README example (#31759)

**Description:** <Describe what has changed.>
<!--Ex. Fixing a bug - Describe the bug and how this fixes the issue.
Ex. Adding a feature - Explain what this achieves.-->
This PR amends the example in the oauth2clientauth extension's README so
that it works out of the box.

**Link to tracking Issue:** <Issue number if applicable> None filed.

**Testing:** <Describe what testing was performed and which tests were
added.> No tests were added

**Documentation:** <Describe the documentation added.> Indented the
ca_file field under a tls block.

Signed-off-by: Paschalis Tsilias <[email protected]>

Author: tpaschalis

extension/oauth2clientauthextension/README.md

@@ -57,7 +57,8 @@ exporters:
 
   otlp/withauth:
     endpoint: 0.0.0.0:5000
-    ca_file: /tmp/certs/ca.pem
+    tls:
+      ca_file: /tmp/certs/ca.pem
     auth:
       authenticator: oauth2client