Clarify the issue number 'Fix' format in pull request

[kdestasio]

Describe the bug
It is not clear that the issue number needs to go next to 'Fix #' without a space in the text of a pull request.

To Reproduce
Steps to reproduce the behavior:

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2. use option '...'
3. See error

Expected behavior
Add instructions to the pull request template that indicate the # and issue number are not separated by a space.

Desktop (please complete the following information):

• OS: [e.g. iOS]
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  nibetaseries version
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[kristianeschenburg]

I'll look at this issue.

[jdkent]

Thanks for bringing up this issue @kdestasio! Would changing Fixes # . to Fixes #<NUM>. (e.g. Fixes #58.) clarify the template?