fix line numbers in a doctest

Author: tkelman

doc/manual/strings.rst

@@ -554,7 +554,7 @@ contained in a string:
     julia> contains("Xylophon", 'o')
     ERROR: MethodError: no method matching contains(::String, ::Char)
     Closest candidates are:
-      contains(!Matched::Function, ::Any, !Matched::Any) at reduce.jl:495
+      contains(!Matched::Function, ::Any, !Matched::Any) at reduce.jl:504
       contains(::AbstractString, !Matched::AbstractString) at strings/search.jl:310
      ...