Fix: Update incorrect client ID in mcp/weather/oauth2 README.md

[BEOKS]

An error was occurring due to an incorrect client ID in README.md. This has been updated to the correct client ID specified in application.yaml.

AS-ID

curl -XPOST "http://localhost:8080/oauth2/token" \
  --data grant_type=client_credentials \
  --user "oidc-client:secret"
{"error":"invalid_client"}

TO-BE

curl -XPOST "http://localhost:8080/oauth2/token" \
--data grant_type=client_credentials \
--user "mcp-client:secret"
{"access_token":"...