Fix typo

(cherry picked from commit ed07eb2)
ref #14062

doc/manual/strings.rst

@@ -692,7 +692,7 @@ You can extract the following info from a :obj:`RegexMatch` object:
 For when a capture doesn't match, instead of a substring, ``m.captures``
 contains ``nothing`` in that position, and ``m.offsets`` has a zero
 offset (recall that indices in Julia are 1-based, so a zero offset into
-a string is invalid). Here's is a pair of somewhat contrived examples::
+a string is invalid). Here is a pair of somewhat contrived examples::
 
     julia> m = match(r"(a|b)(c)?(d)", "acd")
     RegexMatch("acd", 1="a", 2="c", 3="d")