Fix invalid argument in README example for loadtest_config.py (#26277)
Fix invalid argument in README example for loadtest_config.py (#26277)

This example crashes the driver process, since `bq_upload_result.py` expects
the `big_query_table` argument to have the format `<dataset name>.<table name>`.
The project ID is already encoded in the script.

```
+ python3 /src/code/tools/run_tests/performance/bq_upload_result.py --bq_result_table=grpc-testing.e2e_benchmarks.experimental_results_hork_x5
Traceback (most recent call last):
  File "/src/code/tools/run_tests/performance/bq_upload_result.py", line 177, in <module>
    dataset_id, table_id = args.bq_result_table.split('.', 2)
ValueError: too many values to unpack (expected 2)
```
donnadionne/grpc,ctiller/grpc,jtattermusch/grpc,stanley-cheung/grpc,jtattermusch/grpc,nicolasnoble/grpc,donnadionne/grpc,ctiller/grpc,nicolasnoble/grpc,ctiller/grpc,grpc/grpc,vjpai/grpc,grpc/grpc,ctiller/grpc,nicolasnoble/grpc,ejona86/grpc,donnadionne/grpc,ejona86/grpc,ctiller/grpc,ejona86/grpc,ejona86/grpc,ejona86/grpc,vjpai/grpc,vjpai/grpc,ejona86/grpc,ctiller/grpc,donnadionne/grpc,vjpai/grpc,nicolasnoble/grpc,nicolasnoble/grpc,grpc/grpc,stanley-cheung/grpc,ctiller/grpc,ctiller/grpc,donnadionne/grpc,ctiller/grpc,vjpai/grpc,stanley-cheung/grpc,jtattermusch/grpc,stanley-cheung/grpc,stanley-cheung/grpc,donnadionne/grpc,stanley-cheung/grpc,nicolasnoble/grpc,donnadionne/grpc,jtattermusch/grpc,nicolasnoble/grpc,grpc/grpc,ejona86/grpc,donnadionne/grpc,grpc/grpc,jtattermusch/grpc,stanley-cheung/grpc,grpc/grpc,donnadionne/grpc,jtattermusch/grpc,donnadionne/grpc,nicolasnoble/grpc,grpc/grpc,vjpai/grpc,jtattermusch/grpc,jtattermusch/grpc,ctiller/grpc,jtattermusch/grpc,stanley-cheung/grpc,ctiller/grpc,grpc/grpc,vjpai/grpc,jtattermusch/grpc,nicolasnoble/grpc,vjpai/grpc,stanley-cheung/grpc,ejona86/grpc,nicolasnoble/grpc,jtattermusch/grpc,vjpai/grpc,grpc/grpc,vjpai/grpc,donnadionne/grpc,ejona86/grpc,nicolasnoble/grpc,ejona86/grpc,stanley-cheung/grpc,grpc/grpc,stanley-cheung/grpc,jtattermusch/grpc,vjpai/grpc,ejona86/grpc,grpc/grpc,ctiller/grpc,grpc/grpc,ejona86/grpc,stanley-cheung/grpc,nicolasnoble/grpc,donnadionne/grpc,vjpai/grpc