Update client.rst (pymodbus-dev#2199)

Author: qili-eaton

doc/source/client.rst

@@ -15,7 +15,7 @@ It is allowed to have multiple client objects that e.g. each communicate with a
 
 Client performance
 ------------------
-There are currently a big performance gab between the 2 clients
+There are currently a big performance gap between the 2 clients
 (try it on your computer :github:`examples/client_performance.py`).
 This is due to a rather old implementation of the synchronous client, we are currently working to update the client code.
 Our aim is to achieve a similar data rate with both clients and at least double the data rate while keeping the stability.