2.0 migration guide is from 1.7+ to 2.0

The branches have split off at 1.7 so we do not need to
mention 1.8 (or above) explicitly, adding a + there
ought to work out the meaning well enough.

doc/src/guide/migrating_from_1.7.asciidoc

@@ -1,5 +1,5 @@
 [appendix]
-== Migrating from Ranch 1.7 to Ranch 2.0
+== Migrating from Ranch 1.7+ to Ranch 2.0
 
 Ranch 2.0 adds support for multiple connection supervisors.