[DOC] Fix typo in example code for String#encode method

The example code in the documentation for the `String#encode` method has
a typo in the `fallback` option. The example code uses `h` as the
fallback option, but it should use `hash` instead to match the variable
name in the example code.

doc/encodings.rdoc

@@ -419,7 +419,7 @@ These keyword-value pairs specify encoding options:
 
     hash = {"\u3042" => 'xyzzy'}
     hash.default = 'XYZZY'
-    s.encode('ASCII', fallback: h) # => "xyzzyfooXYZZY"
+    s.encode('ASCII', fallback: hash) # => "xyzzyfooXYZZY"
 
     def (fallback = "U+%.4X").escape(x)
       self % x.unpack("U")