Fixed mistake in README

README.md

@@ -32,8 +32,8 @@ json.decode('[1,2,3,{"x":10}]') -- Returns { 1, 2, 3, { x = 10 } }
 
 ## Notes
 * Trying to encode values which are unrepresentable in JSON will never result
-  in type conversion or other magic: sparse arrays, tables with mixed keys or
-  invalid numbers (NaN, -inf, inf) will raise an error
+  in type conversion or other magic: sparse arrays, tables with mixed key types
+  or invalid numbers (NaN, -inf, inf) will raise an error
 * `null` values contained within an array or object are converted to `nil` and
   are therefore lost upon decoding
 * *Pretty* encoding is not supported, `json.encode()` only encodes to a compact