Improve clarity on the number of release signing keys

[AndySchroder]

On the page 2017-01-01-download.md (https://bitcoincore.org/en/download/) the following phrase suggests that there is a single release signing key,

verify_checksums_file: "Verify that the checksums file is PGP signed by the release signing key:"

but the surrounding lines indicate that there are multiple signing keys. I suggest it be changed to

verify_checksums_file: "Verify that the checksums file is PGP signed by trusted keys:"

to avoid confusion.

[superstreber3]

I think this references this sentence in part 8

The output of the command above should say that one key was imported, updated, has new signatures, or remained unchanged.

It is always said that it is recommended to use several keys, but in the example one is imported. That's why I think we're talking about a key

[AndySchroder]

What you say makes sense, but the sentence I've pointed out seems to make it a little contradictory. What about instead changing to

verify_checksums_file: "Verify that the checksums file is PGP signed by a trusted release signing key:"

OR

verify_checksums_file: "Verify that the checksums file is PGP signed by at least one trusted release signing key:"

?

[superstreber3]

I agree, I created an PR (#894) with the change.