doc: fix typo for named regex capture group (#30237)

(cherry picked from commit d8558ee)

doc/src/manual/strings.md

@@ -854,7 +854,7 @@ julia> m[2]
 Captures can be referenced in a substitution string when using [`replace`](@ref) by using `\n`
 to refer to the nth capture group and prefixing the substitution string with `s`. Capture group
 0 refers to the entire match object. Named capture groups can be referenced in the substitution
-with `g<groupname>`. For example:
+with `\g<groupname>`. For example:
 
 ```jldoctest
 julia> replace("first second", r"(\w+) (?<agroup>\w+)" => s"\g<agroup> \1")