README.md tweaks

[ucpete]

All pedantic:

• “IPUAC” -> “IUPAC” typo
• added four missing ambiguous nucleotides to parenthetical list to match claim of ‘all’ being counted
• changed apostrophe after 5 and 3 to prime
• “Plot” -> “Plots” to match tense of other lines
• removed shell from code fence to remove inappropriate highlighting of shell builtins (‘read’, ‘for’, ‘in’, ‘help’, ‘exit’, ‘do’)

[mdshw5]

Thanks!