Update the HTTPX readme to point to proper class method

Author: OlegZv

instrumentation/opentelemetry-instrumentation-httpx/README.rst

@@ -43,11 +43,9 @@ Instrumenting single clients
 ****************************
 
 If you only want to instrument requests for specific client instances, you can
-use the `instrument_client`_ method.
+use the `HTTPXClientInstrumentor.instrument_client` method.
 
 
-.. _instrument_client: #opentelemetry.instrumentation.httpx.HTTPXClientInstrumentor.instrument_client
-
 .. code-block:: python
 
     import httpx