What is the correct formula for diamminechloridonitrito-n-platinum(II)?
So I did some digging, and here's what I found:
According to the current version of Nomenclature of Inorganic Chemistry, IUPAC Recommendations 2005,

IR-9.2.3.1 Sequence of symbols within the coordination formula
(i) The central atom symbol(s) is (are) listed first.
(ii) The ligand symbols (line formulae, abbreviations or acronyms) are then listed in
alphabetical order (see Section IR-4.4.2.2).Thus, CH3CN, MeCN and NCMe would
be ordered under C, M and N respectively, and CO precedes Cl because single letter
symbols precede two letter symbols. The placement of the ligand in the list does not
depend on the charge of the ligand.
(iii) More information is conveyed by formulae that show ligands with the donor atom
nearest the central atom; this procedure is recommended wherever possible, even for
coordinated water.

So in this case, $\ce{Cl}$ should come before $\ce{NH3}$ and the correct formula would be $\ce{[PtCl(NH3)2(NO2)]}$.