doc/manuals: Fix callout definition in code block

Currently osmo-gsm-tester master jenkins build fails while building
manual due to errors like:
"asciidoc: WARNING: test_api.adoc: line 42: no callouts refer to list
item 1"

According to asciidoc documentation in [1], code examples can be
annotated with callouts, but callouts need to be put inside source code
comments.

[1] http://www.methods.co.nz/asciidoc/chunked/ch20.html

Change-Id: I2213874592342b12adf7440b25edc706abfc78ba

doc/manuals/chapters/test_api.adoc

@@ -34,10 +34,10 @@ One can also wait for events in the background, for instance launch a child
 process locally in the same host and wait for its termination:
 [source,python]
 ----
-proc = process.Process('process_description_name', working_dir_to_store_logs, 'sleep 4') <1>
-tenv.remember_to_stop(proc) <2>
-proc.launch() <3>
-proc.wait() <4>
+proc = process.Process('process_description_name', working_dir_to_store_logs, 'sleep 4') # <1>
+tenv.remember_to_stop(proc) # <2>
+proc.launch() # <3>
+proc.wait() # <4>
 ----
 <1> Create process object. This line doesn't yet runs it.
 <2> Make sure the core will kill the process if this test fails