Update us_atrocities.md, improve 1980s Nicaragua paragraph

[orenomfi]

This PR includes improvements to the "From 1982-89, The U.S. government attempted to topple the government of Nicaragua..." paragraph.

• changed the years from 1982-89 to 1981-90, which I think are more factual
• minor typo/grammar fixes
• removed the Honduras link
• added information, especially about the ICJ ruling which is highly significant
• added the 30k casualty number
• more links to sources

[dessalines]

Thx!