Upgrade regex-syntax to 0.6.28

This project was upgraded with external_updater.
Usage: tools/external_updater/updater.sh update rust/crates/regex-syntax
For more info, check https://cs.android.com/android/platform/superproject/+/master:tools/external_updater/README.md

Test: TreeHugger
Change-Id: I93a9c603b72478aaa74119ffece25980544a6ec3

README.md

diff --git a/README.md b/README.md
index e904601..592f842 100644
--- a/README.md
+++ b/README.md
@@ -2,9 +2,8 @@
 ============
 This crate provides a robust regular expression parser.
 
-[![Build status](https://travis-ci.com/rust-lang/regex.svg?branch=master)](https://travis-ci.com/rust-lang/regex)
-[![Build status](https://ci.appveyor.com/api/projects/status/github/rust-lang/regex?svg=true)](https://ci.appveyor.com/project/rust-lang-libs/regex)
-[![](https://meritbadge.herokuapp.com/regex-syntax)](https://crates.io/crates/regex-syntax)
+[![Build status](https://github.com/rust-lang/regex/workflows/ci/badge.svg)](https://github.com/rust-lang/regex/actions)
+[![Crates.io](https://img.shields.io/crates/v/regex-syntax.svg)](https://crates.io/crates/regex-syntax)
 [![Rust](https://img.shields.io/badge/rust-1.28.0%2B-blue.svg?maxAge=3600)](https://github.com/rust-lang/regex)
 
 
@@ -53,7 +52,7 @@
 
 The standard for using `unsafe` in this crate is extremely high because this
 crate is intended to be reasonably safe to use with user supplied regular
-expressions. Therefore, while their may be bugs in the regex parser itself,
+expressions. Therefore, while there may be bugs in the regex parser itself,
 they should _never_ result in memory unsafety unless there is either a bug
 in the compiler or the standard library. (Since `regex-syntax` has zero
 dependencies.)