Fix mentioned Python versions to headline (#333)

In the previous version, the document gave an example on how to compare Python 3.6 and Python 3.7 performance.
However, the commands in the example referred partially to Python 3.8.
Since this is confusing and not inline with the first line of the section, I changed the example so that it fits the text.

Author: HelgeCPH

doc/usage.rst

@@ -51,11 +51,11 @@ Run benchmarks
 Commands to compare Python 3.6 and Python 3.7 performance::
 
     pyperformance run --python=python3.6 -o py36.json
-    pyperformance run --python=python3.7 -o py38.json
-    pyperformance compare py36.json py38.json
+    pyperformance run --python=python3.7 -o py37.json
+    pyperformance compare py36.json py37.json
 
 Note: ``python3 -m pyperformance ...`` syntax works as well (ex: ``python3 -m
-pyperformance run -o py38.json``), but requires to install pyperformance on each
+pyperformance run -o py37.json``), but requires to install pyperformance on each
 tested Python version.
 
 JSON files are produced by the pyperf module and so can be analyzed using pyperf
@@ -71,7 +71,7 @@ commands::
 
 It's also possible to use pyperf to compare results of two JSON files::
 
-    python3 -m pyperf compare_to py36.json py38.json --table
+    python3 -m pyperf compare_to py36.json py37.json --table
 
 Basic commands
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