yaml_load should not be B5xx cryptography group

[ericwb]

Describe the bug
The yaml_load plugin has bandit ID B506. The 5xx group according to [1] is defined as the group for cryptography. This plugin would be more appropriate as a type of injection B6xx

To Reproduce
n/a

Expected behavior
n/a

Bandit version

bandit 1.4.0

Additional context
Add any other context about the problem here.

[ehooo]

I created the issue #293 for add on README the explanation for each code,
I think this could improve the new codes for contributors.