fix(docs-contributing.rst): fix formatting for code snippet

This patch will fix the formatting errors concerning code snippet,
lines 245 and 256 respectively.
The code snippet is updated to 'shell' to lex it appropriately.

Signed-off-by: Jayanth Dodderi Chidanand <jayanthdodderi.chidanand@arm.com>
Change-Id: I53aefd81da350b6511e7a97b5fee7b0d6f9dde2d
1 file changed