bug#24407: 24.5; doc of `format'

[Eli Zaretskii]

> Date: Sat, 10 Sep 2016 15:20:25 -0700 (PDT)
> From: Drew Adams <drew.adams@oracle.com>
> 
> The Elisp manual doc for `format' (node Formatting Strings) looks OK.
> But the doc string has this bug:
> 
>   where flags is [+ #-0]+, width is [0-9]+, and precision is .[0-9]+
> 
>                                   must be a literal `.' here ^

Yes, using a period in a regexp-like expression is confusing, when we
mean a literal period.

> The precision part should be this (or equivalent): "[.][0-9]+", not
> ".[0-9]+".

But this is not the best solution.  I opted for telling that precision
is a literal period followed by [0-9]+ instead.

Thanks, fixed on master.