Add reference to indent function (#38940)

* Add reference to indent function

* Update base/strings/io.jl

Co-authored-by: Steven G. Johnson <stevenj@alum.mit.edu>

Co-authored-by: Keno Fischer <keno@juliacomputing.com>
Co-authored-by: Steven G. Johnson <stevenj@alum.mit.edu>

Author: 3 people

base/strings/io.jl

@@ -633,6 +633,8 @@ julia> Base.unindent("   a\\n   b", 2)
 julia> Base.unindent("\\ta\\n\\tb", 2, tabwidth=8)
 "      a\\n      b"
 ```
+
+See also `indent` from the [`MultilineStrings` package](https://github.com/invenia/MultilineStrings.jl).
 """
 function unindent(str::AbstractString, indent::Int; tabwidth=8)
     indent == 0 && return str