docs: add missing apostrophes (#1344)

lib/chai/core/assertions.js

@@ -944,7 +944,7 @@ module.exports = function (chai, _) {
    *
    * Add `.not` earlier in the chain to negate `.arguments`. However, it's often
    * best to assert which type the target is expected to be, rather than
-   * asserting that its not an `arguments` object.
+   * asserting that it’s not an `arguments` object.
    *
    *     expect('foo').to.be.a('string'); // Recommended
    *     expect('foo').to.not.be.arguments; // Not recommended
@@ -1926,7 +1926,7 @@ module.exports = function (chai, _) {
    * a `descriptor`. The problem is that it creates uncertain expectations by
    * asserting that the target either doesn't have a property descriptor with
    * the given key `name`, or that it does have a property descriptor with the
-   * given key `name` but its not deeply equal to the given `descriptor`. It's
+   * given key `name` but it’s not deeply equal to the given `descriptor`. It's
    * often best to identify the exact output that's expected, and then write an
    * assertion that only accepts that exact output.
    *