Apply new libdmtx shared library name on linux for readme.md file

[sayman486]

Before beginning I appriciate your works.

conclusion: libdmtx0a -> libdmtx0b for linux users

I used ubuntu 22 lts and when I import pylibdmtx lib I got a error because I didn't install libdmtx package.

but when I exucete command from readme.md file below

sudo apt-get install libdmtx0a

then I got a error message like below

Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
Package libdmtx0a is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
However the following packages replace it:
  libdmtx0b

after I modified libdmtx0a to libdmtx0b then I succes import pylibdmtx.pylibdmtx in python3.