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.
Merge pull request #30 from etripier/Fix-minor-typo-in-README.md

Fix minor typo in README.md