syntax/doc: fix 'their' typo

Author: isker

regex-syntax/README.md

@@ -52,7 +52,7 @@ for extreme optimization, and therefore, use of `unsafe`.
 
 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.)