implement review feedback by removing code comment and MD in error msg

Signed-off-by: tarilabs <matteo.mortari@gmail.com>

Author: tarilabs

oras/auth/ecr.py

@@ -53,15 +53,11 @@ def authenticate_request(
                 logger.warning(f"realm: {h.realm} did not match expected pattern.")
                 return super().request_token(h)
             region = m.group("region")
-            try:  # avoid mandatory requirement on `boto3`
+            try:
                 import boto3
             except ImportError as e:
                 msg = """the `boto3` dependency is required to support authentication to this registry.
-                Make sure to install the required extra, e.g.:
-
-                ```sh
-                pip install oras[ecr]
-                ```
+                Make sure to install the required extra "ecr", e.g.: pip install oras[ecr].
                 """
                 raise ImportError(msg) from e
             ecr = boto3.client("ecr", region_name=region)