clarify Key is a string in README.md re #50

Author: andreineculau

README.md

@@ -55,15 +55,15 @@ There are two ways of using jesse:
 ```erlang
 1> Schema = jiffy:decode(<<"{\"items\": {\"type\": \"integer\"}}">>).
 {[{<<"items">>,{[{<<"type">>,<<"integer">>}]}}]}
-2> jesse:add_schema(some_key, Schema).
+2> jesse:add_schema("some_key", Schema).
 ok
 3> Json1 = jiffy:decode(<<"[1, 2, 3]">>).
 [1,2,3]
-4> jesse:validate(some_key, Json1).
+4> jesse:validate("some_key", Json1).
 {ok,[1,2,3]}
 5> Json2 = jiffy:decode(<<"[1, \"x\"]">>).
 [1,<<"x">>]
-6> jesse:validate(some_key, Json2).
+6> jesse:validate("some_key", Json2).
 {error,[{data_invalid,{[{<<"type">>,<<"integer">>}]},
                       wrong_type,<<"x">>,
                       [1]}]}
@@ -75,15 +75,15 @@ validation failed. See *Validation errors* below for the full error format.
 (using a callback)
 
 ```erlang
-1> jesse:add_schema(some_key,
+1> jesse:add_schema("some_key",
 1>                  <<"{\"uniqueItems\": true}">>,
 1>                  [{parser_fun, fun jiffy:decode/1}]).
 ok
-2> jesse:validate(some_key,
+2> jesse:validate("some_key",
 2>                <<"[1, 2]">>,
 2>                [{parser_fun, fun jiffy:decode/1}]).
 {ok,[1, 2]}
-3> jesse:validate(some_key,
+3> jesse:validate("some_key",
 3>                <<"[{\"foo\": \"bar\"}, {\"foo\": \"bar\"}] ">>,
 3>                [{parser_fun, fun jiffy:decode/1}]).
 {error,[{data_invalid,{[{<<"uniqueItems">>,true}]},