gpg: fixup README

trezor_agent/gpg/README.md

@@ -1,23 +1,23 @@
 # Generate new GPG signing key:
 ```
-$ USER_ID="Satoshi Nakamoto <satoshi@nakamoto.bit>"
+$ export TREZOR_GPG_USER_ID="Satoshi Nakamoto <satoshi@nakamoto.bit>"
 ```
 
 ## Create new GPG identity:
 ```
 $ trezor-gpg create > identity.pub                  # create new TREZOR-based GPG identity
 $ gpg2 --import identity.pub                        # import into local GPG public keyring
-$ gpg2 --edit "${USER_ID}" trust                    # OPTIONAL: mark the key as trusted
+$ gpg2 --edit "${TREZOR_GPG_USER_ID}" trust         # OPTIONAL: mark the key as trusted
 ```
 
-# Create new subkey for an existing GPG identity:
+## Create new subkey for an existing GPG identity:
 ```
-$ gpg2 --list-keys "${USER_ID}"                     # make sure this identity already exists
+$ gpg2 --list-keys "${TREZOR_GPG_USER_ID}"          # make sure this identity already exists
 $ trezor-gpg create --subkey > identity.pub         # create new TREZOR-based GPG public key
 $ gpg2 --import identity.pub                        # append it to existing identity
 ```
 
-# Generate signatures using the TREZOR device
+# Generate GPG signatures using a TREZOR device:
 ```
 $ trezor-gpg sign EXAMPLE                           # confirm signature using the device
 $ gpg2 --verify EXAMPLE.asc                         # verify using standard GPG binary