fix(docs-contributing.rst): fix formatting for code snippet
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 <8c3a0fb3113ec76e03e9c622faea755797783ca0@arm.com>
Change-Id: I53aefd81da350b6511e7a97b5fee7b0d6f9dde2d

achingupta/arm-trusted-firmware,achingupta/arm-trusted-firmware