Fix minor type in README.md

Fixes minor typo on line 132.

Before: Designed to inter-operate with your existing JavaScript, `immutable` accepts plain JavaScript [Array] and Objects anywhere a method expects a `Sequence` with no performance penalty.

After: Designed to inter-operate with your existing JavaScript, `immutable` accepts plain JavaScript [Arrays] and Objects anywhere a method expects a `Sequence` with no performance penalty.

This corrects the parallel structure of the clause and slightly improves readability. I'd like to propose additional changes to document grammar and syntax, if desired.