Fixes and improvements to Git Preparation page

[emiliom]

• Fixed inconsistent, mixed use of master and main
• Settled on a single name for the target repo name (two were being used)
• Other small fixes
• Turned the lead in to step 6 (merge conflict) into a more prominent admonition

[netlify[bot]]

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[emiliom]

This is a straightforward set of fixes to one page. So, I'll self merge.