docs: fix simple typo, portible -> portable (#547)

There is a small typo in tests/dbapi20.py.

Should read `portable` rather than `portible`.

tests/dbapi20.py

@@ -56,7 +56,7 @@
 # - self.populate is now self._populate(), so if a driver stub
 #   overrides self.ddl1 this change propagates
 # - VARCHAR columns now have a width, which will hopefully make the
-#   DDL even more portible (this will be reversed if it causes more problems)
+#   DDL even more portable (this will be reversed if it causes more problems)
 # - cursor.rowcount being checked after various execute and fetchXXX methods
 # - Check for fetchall and fetchmany returning empty lists after results
 #   are exhausted (already checking for empty lists if select retrieved