Fix add/remove_index detail in README

[henrahmagix]

Both add_index and remove_index use without_timeout(&), so they don't set a lock timeout of 30 seconds =)

See

safe-pg-migrations/lib/safe-pg-migrations/plugins/statement_insurer.rb

Line 64 in b0efa6d

without_timeout { super(table_name, column_name, **options) }

and

safe-pg-migrations/lib/safe-pg-migrations/plugins/statement_insurer.rb

Line 71 in b0efa6d

without_timeout { super(table_name, column_name, **options) }