Change fetch command to pull command

[Surendra Pathak]

Change the fetch command to pull command to match related services. Here are the changes:

1. command name in the help and description changed as follows

build/sbomex --help

....
fetch Downloads specified SBOM from the repository and prints to the screen
pull Pulls specified SBOM from the repository and prints to the screen

2. build/sbomex pull --help
   Pulls specified SBOM from the repository and prints to the screen

Usage:
sbomex pull [flags]

Flags:
[Remove this one for now] --filter string Filter SBOM based on conditions provided
-h, --help help for pull
--id int Pull SBOM based on the input Id

3. Update README.md - @surendrapathak is on this.