Confusing wording for patch "id" properties

[lionello]

According to the spec, the "id" properties for the add-public-keys and add-services patches are "Base64url encoded", but are not actually being encoded at all. Probably what was meant was to restrict the allowed characters to be the ones from the URL-safe "Base64url" character set i.e. lower case, upper case, digits, _ and -.

https://github.com/decentralized-identity/sidetree/blame/master/docs/spec/patches.md#L40

    1. The object ****MUST**** include an `id` property, and its value ****MUST**** be a string with no more than fifty (50) Base64URL encoded characters. If the value is not of the correct type or exceeds the specified maximum length, the entire _Patch Action_ ****MUST**** be discarded, without any of the patch being used to modify the DID's state.

https://github.com/decentralized-identity/sidetree/blame/master/docs/spec/patches.md#L143

    1. The object ****MUST**** include an `id` property, and its value ****MUST**** be a string with a length of no more than fifty (50) Base64URL encoded characters. If the value is not of the correct type or exceeds the specified length, the entire _Patch Action_ ****MUST**** be discarded, without any of it being used to modify the DID's state.

(Additionally, the two specs for the two id properties are very similar, differing only by a length of in the first sentence. This could also be made to match for both occurrences.)

[lionello]

The URL Safe Base 64 Alphabet is spec'ed in Table 2 in https://tools.ietf.org/html/rfc4648#section-5

[decentralgabe]

To be done with #1208