Update grammar in BuildInstructions.md

[Typecosa]

Changes to improve grammar and spelling in the file

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This change is 

[MarkusPettersson98]

Looks pretty good! Left a small nit ⛏️

Also, we can't accept PRs that merge main into them. Please rebase on top of main instead. Drop the last merge commit and run git rebase origin main where origin is the upstream repository git@github.com:mullvad/mullvadvpn-app.git. Thanks!

Reviewed 1 of 1 files at r1, all commit messages.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @Typecosa)

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BuildInstructions.md line 144 at r1 (raw file):

./build.sh [--optimize]

This should produce an installer exe, pkg, or rpm+deb file in the dist/ directory.

We don't have any rules stating that we should use Oxford commas. Please, revert this change 😊

Code quote:

exe, pkg, or rpm+deb