Update images directory

[QuanDool]

The images were not loaded as the images folder has been moved up one level. After the change, the images are now visible.

[atishp04]

The changes won't work as the the Makefile builds the images from the parent directory. Here are the build errors

asciidoctor: WARNING: image to embed not found or not readable: /home/atish/workspace/specs/images/riscv-sbi-intro1.png
asciidoctor: WARNING: image to embed not found or not readable: /home/atish/workspace/specs/images/riscv-sbi-intro2.png
asciidoctor -d book -b html riscv-sbi.adoc
asciidoctor: WARNING: optional gem 'pygments.rb' is not available (reason: cannot load 'pygments'). Functionality disabled.
asciidoctor: WARNING: image to embed not found or not readable: /home/atish/workspace/specs/images/riscv-sbi-intro1.png
asciidoctor: WARNING: image to embed not found or not readable: /home/atish/workspace/specs/images/riscv-sbi-intro2.png