doc: board: j721e_evm: Fix code-block type and indents

* BASH code should be labeled as such.
 * Code blocks should be indented by 4 spaces.

Fix these here.

Suggested-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Andrew Davis <afd@ti.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>