docs : Fix duplicated file extension in test command (#11935)

This commit fixes an issue in the llama.cpp project where the command for testing the llama-server object contained a duplicated file extension. The original command was:
./tests.sh unit/test_chat_completion.py.py -v -x
It has been corrected to:
./tests.sh unit/test_chat_completion.py -v -x
This change ensures that the test script correctly locates and executes the intended test file, preventing test failures due to an incorrect file name.

Author: xiaobing318

examples/server/tests/README.md

@@ -48,7 +48,7 @@ DEBUG=1 ./tests.sh -s -v -x
 To run all the tests in a file:
 
 ```shell
-./tests.sh unit/test_chat_completion.py.py -v -x
+./tests.sh unit/test_chat_completion.py -v -x
 ```
 
 To run a single test: