[webauthn] capitalize "must" in Note in authenticatorGetAssertion operation

equalsJeffH has just created a new issue for https://github.com/w3c/webauthn:

== capitalize "must" in Note in authenticatorGetAssertion operation ==
the initial Note in [S 6.2.3. The authenticatorGetAssertion operation](https://w3c.github.io/webauthn/#op-get-assertion) says:
> Before performing this operation, all other operations in progress in the authenticator session must be aborted by running the authenticatorCancel operation.

in the similar Note up in [6.2.2. The authenticatorMakeCredential operation](https://w3c.github.io/webauthn/#op-make-cred) the must is "MUST"

they should both be capitalized or lower case -- I'm thinking capitalized.

Please view or discuss this issue at https://github.com/w3c/webauthn/issues/938 using your GitHub account