Minor change to get output file name

[Jousimies]

on line

plantuml-emacs/plantuml.el

Line 137 in daeb3c3

(file-name-sans-extension (buffer-name))

use (file-name-sans-extension (buffer-name)) to generate output file name.

I temp find buffer-name could not be the right file name, I suggest use (file-name-base (buffer-file-name)) to get file name.