Small typo fixes and additions

[nasbench]

This PR adds/updates the following:

• Fixes some typos and messages wording.
• Reorders the modifier_mapping dict for ease of reading.
• Adds a new related type CORRELATION as suggested by Add correlation (or similar) related type sigma-specification#136. (The specification part is in progress). I think its an easy and useful add.
• Remove the date check for dates yyyy/mm/dd in the SigmaRulesBase class to conform with the specs v2
• Add exception handling for parsing sigma validation config file. (in case the file provided isn't a proper yaml)

[frack113]

LGTM