Use single quotes for allow_invalid_escape doc

Instead of using %{} which works like double-quoted string and allows
escape sequences and in which "other escaped characters (a backslash
followed by a character) are interpreted as the character", change the
examples for `allow_invalid_escape` to use single-quotes strings, in
which "any other character following a backslash [except ' and \] is
interpreted as is: a backslash and the character itself."

This makes it clearer the JSON document only includes a single
backslash.

Author: etiennebarrie

lib/json.rb

@@ -201,10 +201,10 @@
 # defaults to +false+.
 #
 # With the default, +false+:
-#   JSON.parse(%{"Hell\\o"}) # invalid escape character in string (JSON::ParserError)
+#   JSON.parse('"Hell\o"') # invalid escape character in string (JSON::ParserError)
 #
 # When enabled:
-#   JSON.parse(%{"Hell\\o"}, allow_invalid_escape: true) # => "Hello"
+#   JSON.parse('"Hell\o"', allow_invalid_escape: true) # => "Hello"
 #
 # ====== Output Options
 #