Clarify wording of PR & issue templates

I see lots of contributions where none of this placeholder text has
been deleted, but also quite a lot where the first line has been deleted,
but the 'ext below' hasn't (exactly in accordance with the current
instructions).

.github/ISSUE_TEMPLATE.md

-Please delete the text below before submitting your contribution. 
+Please delete this line and the text below before submitting your contribution.
 
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.github/PULL_REQUEST_TEMPLATE.md

-Please delete the text below before submitting your contribution. 
+Please delete this line and the text below before submitting your contribution.
 
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