Update cache documentation example to correctly show cache hit and miss

Author: hyunchel

changelog/4558.doc.rst

@@ -0,0 +1 @@
+Update cache documentation example to correctly show cache hit and miss.

doc/en/cache.rst

@@ -185,20 +185,22 @@ across pytest invocations::
     import pytest
     import time
 
+    def expensive_computation():
+        print("running expensive computation...")
+
     @pytest.fixture
     def mydata(request):
         val = request.config.cache.get("example/value", None)
         if val is None:
-            time.sleep(9*0.6) # expensive computation :)
+            expensive_computation()
             val = 42
             request.config.cache.set("example/value", val)
         return val
 
     def test_function(mydata):
         assert mydata == 23
 
-If you run this command once, it will take a while because
-of the sleep:
+If you run this command for the first time, you can see the print statement:
 
 .. code-block:: pytest
 
@@ -217,7 +219,7 @@ of the sleep:
     1 failed in 0.12 seconds
 
 If you run it a second time the value will be retrieved from
-the cache and this will be quick:
+the cache and nothing will be printed:
 
 .. code-block:: pytest