Minor change to get output file name

[Jousimies]

on line https://github.com/ginqi7/plantuml-emacs/blob/daeb3c308f08fe76670268f2a9795d77be86c22a/plantuml.el#L137

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.