Correcting a typo.

Should be `'o'` instead of `'0'`.
(cherry picked from commit 9a3c68f)
ref PR #9460

doc/manual/strings.rst

@@ -512,7 +512,7 @@ contained in a string:
  julia> contains("Xylophon", 'o')
  ERROR: `contains` has no method matching contains(::ASCIIString, ::Char)
 
-The last error is because ``'0'`` is a character literal, and :func:`contains`
+The last error is because ``'o'`` is a character literal, and :func:`contains`
 is a generic function that looks for subsequences. To look for an element in a
 sequence, you must use the :func:`in` function instead.