Update text copy for corpus management

What does this MR do and why?

Text copy changes for #352470 (closed)

Before

to "Corpus files are used in coverage-guided fuzz testing as seed inputs to improve testing."

After

How to set up and validate locally

• From the gitlab repo open a rails console and enable the :corpus_management AND :corpus_management_ui feature flag

bundle exec rails c

then

[4] pry(main)> Feature.enable(:corpus_management)
[4] pry(main)> Feature.enable(:corpus_management_ui)

Navigate to:

http://172.16.123.1:3000///-/security/configuration/

and

http://172.16.123.1:3000///-/security/configuration/corpus_management