Update command line help baseline report

The command line help for the baseline parameter is misleading.
The passed output has to be JSON, but the output formatters for
the result are different for baseline (another command line option).

Change-Id: I1f6d760af96b48472027dba94e585872768103c8
Closes-Bug: #1534358

README.rst

@@ -126,9 +126,8 @@ Usage::
                             Note that these are in addition to the excluded paths
                             provided in the config file.
       -b BASELINE, --baseline BASELINE
-                            Path to a baseline report, in JSON format. Note:
-                            baseline reports must be output in one of the
-                            following formats: ['html', 'json', 'screen', 'txt']
+                            Path to a baseline report. Only JSON formatted files
+                            are accepted.
       --ini INI_PATH        Path to a .bandit file which supplies command line
                             arguments to Bandit.
       --version             show program's version number and exit

bandit/cli/main.py

@@ -232,10 +232,8 @@ def main():
     )
     parser.add_argument(
         '-b', '--baseline', dest='baseline', action='store',
-        default=None, help=('Path to a baseline report, in JSON format. '
-                            'Note: baseline reports must be output in one of '
-                            'the following formats: ' +
-                            str(sorted(baseline_formatters)))
+        default=None, help=('Path to a baseline report. Only JSON formatted '
+                            'files are accepted.')
     )
     parser.add_argument(
         '--ini', dest='ini_path', action='store', default=None,