Update documentation.md (#22121)

doc/src/manual/documentation.md

@@ -110,7 +110,7 @@ As in the example above, we recommend following some simple conventions when wri
        produce consistent outputs during different Julia sessions.
 
        Operating system word size ([`Int32`](@ref) or [`Int64`](@ref)) as well as path separator differences
-       (`/` or `\`) will also effect the reproducibility of some doctests.
+       (`/` or `\`) will also affect the reproducibility of some doctests.
 
        Note that whitespace in your doctest is significant! The doctest will fail if you misalign the
        output of pretty-printing an array, for example.