Fix doctests for CodeUnits indexing. (#44105)

Author: fredrikekre

doc/src/manual/strings.md

@@ -1088,7 +1088,7 @@ julia> x[1]
 0x31
 
 julia> x[1] = 0x32
-ERROR: setindex! not defined for Base.CodeUnits{UInt8, String}
+ERROR: CanonicalIndexError: setindex! not defined for Base.CodeUnits{UInt8, String}
 [...]
 
 julia> Vector{UInt8}(x)