fix UCBadmit demo in README.md

[tjmahr]

The reject column leads to a Stan error: Identifier 'reject' clashes with reserved keyword.

Compiling Stan program...
Semantic error in 'C:/Users/Tristan/AppData/Local/Temp/RtmpANG1E7/model-43e43a135551.stan', line 3, column 18 to column 24:
   -------------------------------------------------
     1:  data{
     2:      array[12] int male;
     3:      array[12] int reject;
                           ^
     4:      array[12] int applications;
     5:      array[12] int admit;
   -------------------------------------------------

Identifier 'reject' clashes with reserved keyword.

Idk how much you rely on this variable name in the book, but this is a quick fix for the README.