Remove ineffective regexes from docs

[emostov]

The regexp says "A string of length between 8 to 64 composed of: numbers 1 through 9 and any lower- or higher case letter, except the following: 'I', 'O', 'l' (lower-case L) and 'o'". Visualized. Another playground.

SS58 account IDs can't be only 8 chars long, can they? And don't they have a prefix depending on the network?

Originally posted by @dvdplm in #533 (comment)