Fix exiftool command in README.md

[brandl-muc]

The README.md suggests to run exiftool with the following command:

exiftool -overwrite_original -r -if 'not defined DateTimeOriginal' -P "-AllDates<FileModifyDate" "your/output/folder/ALL_PHOTOS/"

For me, this always yields n files failed condition.

At first, I shrugged and brushed this off, assuming that all files had the DateTimeOriginal tag. But later, I became suspicious and run tests with two files, one with the tag and one without it. (but both with a CreateDate tag to have something to print)
To cut it short, the current command does not work as expected:

 exiftool -if 'not defined DateTimeOriginal' -CreateDate .
    1 directories scanned
    2 files failed condition
    0 image files read

Actually, the tag name needs a dollar sign as prefix:

% exiftool -if 'not defined $DateTimeOriginal' -CreateDate .
======== ./no_dto.jpg
Create Date                     : 2025:07:02 20:49:00
    1 directories scanned
    1 files failed condition
    1 image files read

For reference, see also exiftool man on -if

EXPR is a Perl-like logic expression containing tag names prefixed by $ symbols.