Fix incorrect calculation

V1E18MF{some_date} means "add 17 months and go to the last day of that month"

We had been calculating "add 18 months and go to the last day of that month"

lib/sof/cycles/end_of.rb

@@ -17,7 +17,7 @@ def to_s
       end
 
       def final_date(_ = nil) = time_span
-        .end_date(start_date)
+        .end_date(start_date - 1.send(period))
         .end_of_month
 
       def start_date(_ = nil) = from_date.to_date

spec/sof/cycles/end_of_spec.rb

@@ -10,7 +10,7 @@ module SOF
     let(:notation) { "V2E18MF#{from_date}" }
     let(:anchor) { nil }
 
-    let(:end_date) { (from_date.to_date + 18.months).end_of_month }
+    let(:end_date) { (from_date.to_date + 17.months).end_of_month }
     let(:from_date) { "2020-01-01" }
 
     let(:completed_dates) do
@@ -38,7 +38,7 @@ module SOF
       end
     end
 
-    @end_date = ("2020-01-01".to_date + 18.months).end_of_month
+    @end_date = ("2020-01-01".to_date + 17.months).end_of_month
     it_behaves_like "#to_s returns",
       "2x by #{@end_date.to_fs(:american)}"
 
@@ -52,7 +52,7 @@ module SOF
     it_behaves_like "#notation returns the notation"
     it_behaves_like "#as_json returns the notation"
     it_behaves_like "it computes #final_date(given)",
-      given: nil, returns: ("2020-01-01".to_date + 18.months).end_of_month
+      given: nil, returns: ("2020-01-01".to_date + 17.months).end_of_month
 
     describe "#covered_dates" do
       it "given an anchor date, returns dates that fall within it's window" do