Flip snap packaging's grade to stable

This change allows downstream packagers to publish the snap to the
candidate and stable channels.

Signed-off-by: 林博仁(Buo-ren, Lin) <Buo.Ren.Lin@gmail.com>

snap/snapcraft.yaml

@@ -6,7 +6,7 @@ description: |
   languages "out of the box". It can be trained to recognize other languages.
   Tesseract supports various output formats: plain-text, hocr(html), pdf.
 
-grade: devel # must be 'stable' to release into candidate/stable channels
+grade: stable # must be 'stable' to release into candidate/stable channels
 confinement: strict
 
 apps: