chore[docs]: Tornado 5/6 support for Python 3.7+ (#2632)

* Fixed documentation error from Python 3.7 to 3.7+

* Fixed unclear comment.

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>
Co-authored-by: Kyle Verhoog <[email protected]>

Author: 3 people

ddtrace/contrib/tornado/__init__.py

@@ -3,7 +3,7 @@
 Auto instrumentation is available using the ``patch`` function that **must be called before**
 importing the tornado library.
 
-**Note:** Tornado 5 and 6 supported only for Python 3.7.
+**Note:** This integration requires Python 3.7 and above for Tornado 5 and 6.
 
 The following is an example::