Fix for covid-vax story: 2nd doses can be given earlier than specified

[jack89roberts]

Summary

Fix a bug in the Covid-19 vaccine forecasting story which meant 2nd doses could be given earlier than the specified min_second_delay value in the forecast_vaccines function (only impacts the latter stages of the forecasts after all adults vaccinated with 1 dose).

List of changes proposed in this PR (pull-request)

All to the stories/covid-vax/covid_vax.ipynb notebook. The completion dates of the forecasts don't change, but I've had to make a few corrections to the text for the SAGE forecast:

• Corrections to the code in the forecast_vaccines function.
• Set min_second_delay=28 as the default for the forecast_sage function.
• Add a paragraph starting "We should also note that we've allowed vaccines to be given sooner than 11 weeks" as the second to last paragraph before the conclusions.
• Modify the 3rd paragraph of the conclusions (starting "As well as the uncertainty in future vaccine supply and demand") to mention the possible limitations with allowing 2nd doses to be given earlier near the end of the programme.

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