[docs] Howto/regex omits the simplest {n} case

Hi, https://docs.python.org/3/howto/regex.html#repeating-things states: *You can omit either m or n; in that case, a reasonable value is assumed for the missing value. Omitting m is interpreted as a lower limit of 0, while omitting n results in an upper bound of infinity.* This is correct, for the cases of {m,} and {,n}. What isn't covered is the simplest case {m} which matches the preceding item exactly m times. Regards, Derek PS https://docs.python.org/3/howto/regex.html is generally one of the best bits of software documentation I have come across: it's a model of simplicity and clarity, on a notoriously difficult topic.