Fix typo in error thrown by Base58.decode()

src/base58.coffee

@@ -16,7 +16,7 @@ class Base58Builder
     num = 0
     for char, index in str.split(//).reverse()
       if (char_index = @alphabet.indexOf(char)) == -1
-        throw new Error('Value passed not a valid Base58 string.')
+        throw new Error('Value passed is not a valid Base58 string.')
       num += char_index * Math.pow(@base, index)
     num