Fixed "sort order" -> "insertion order".

README.rst

@@ -50,14 +50,14 @@ the sdist.
 Compatibility
 -------------
 
-This backport assumes that dict objects retain their sort order.  This
-is true in the language spec for Python 3.7 and greater.  Since this
-is a backport to Python 3.6, it raises an interesting question: does
-that guarantee apply to 3.6?  For CPython 3.6 it does.  As of the time
-of this writing, it's also true for all other Python implementations
-that claim to be 3.6 compatible, of which there are none.  Any new
-3.6 implementations are expected to have ordered dicts.  See the
-analysis at the end of this email:
+This backport assumes that dict objects retain their insertion order.
+This is true in the language spec for Python 3.7 and greater.  Since
+this is a backport to Python 3.6, it raises an interesting question:
+does that guarantee apply to 3.6?  For CPython 3.6 it does.  As of the
+time of this writing, it's also true for all other Python
+implementations that claim to be 3.6 compatible, of which there are
+none.  Any new 3.6 implementations are expected to have ordered dicts.
+See the analysis at the end of this email:
 
 https://mail.python.org/pipermail/python-dev/2017-December/151325.html