Fix Python manual timing example (#1722)

According to the user guide, when manual timing, it is necessary to explicit it by using the `UseManualTime` function. Its equivalent in Python is use_manual_time(). This function was not called in the example.

It is possible to verify that the use of this function has an impact on the measure by adding another `time.sleep(0.01)` at the end of the iteration. There is a x2 difference depending on whether `use_manual_time()` is used or not.

Co-authored-by: dominic <510002+dmah42@users.noreply.github.com>

bindings/python/google_benchmark/example.py

@@ -61,6 +61,7 @@ def skipped(state):
 
 
 @benchmark.register
+@benchmark.option.use_manual_time()
 def manual_timing(state):
     while state:
         # Manually count Python CPU time