Minor change to exam.el

[Mandar Mitra]

When using the exam documentclass, each choice within a choices environment is 
indented like so:

  \begin{choices}
    \choice ....
    |<- continued line starts here right under the \ of \choice

To me, it makes sense to treat \choice like \item, so I'd like to suggest the 
following change around line 127 of exam.el:

*** 127,132 ****
--- 127,134 ----
            (concat
             LaTeX-item-regexp
             "\\|"
+            "choice\\b"
+            "\\|"
             "\\(titled\\)?question\\b"
             "\\|"
             "\\(sub\\|subsub\\)?part\\b"))

Does this make sense? Thanks,
Mandar.