INSTALL.md: fixes to CREATE USER statement

Previously copying the CREATE USER text would cause psql to look for a
closing single-quote. By changing the backtick to a single-quote the
command now works correctly when copying and pasting into psql.

This change also closes the statement with a semicolon.

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It looks like you have an outstanding pull request where I made these changes,
so I figured I'd send this your way instead of upstream to 18F.