Fix reST markup in dataclasses.rst (GH-27843) (GH-27845)

The signature of field() had an extraneous colon at the end, causing it
to appear all bold and without the module name.
(cherry picked from commit d26dbba9297adb59bc49a6583c647804ef56fc58)

Co-authored-by: Jean-Abou-Samra <37271310+Jean-Abou-Samra@users.noreply.github.com>

Co-authored-by: Jean-Abou-Samra <37271310+Jean-Abou-Samra@users.noreply.github.com>