[options] mark '--list-extractors' argument as optional

docs/options.md

@@ -48,13 +48,13 @@
     -K, --list-keywords         Print a list of available keywords and example
                                 values for the given URLs
     -e, --error-file FILE       Add input URLs which returned an error to FILE
-    -N, --print [EVENT:]FORMAT  Write FORMAT during EVENT (default 'prepare') to
-                                standard output. Examples: 'id' or
+    -N, --print [EVENT:]FORMAT  Write FORMAT during EVENT (default 'prepare')
+                                to standard output. Examples: 'id' or
                                 'post:{md5[:8]}'
     --print-to-file [EVENT:]FORMAT FILE
                                 Append FORMAT during EVENT to FILE
     --list-modules              Print a list of available extractor modules
-    --list-extractors CATEGORIES
+    --list-extractors [CATEGORIES]
                                 Print a list of extractor classes with
                                 description, (sub)category and example URL
     --write-log FILE            Write logging output to FILE
@@ -67,7 +67,8 @@
 
 ## Networking Options:
     -R, --retries N             Maximum number of retries for failed HTTP
-                                requests or -1 for infinite retries (default: 4)
+                                requests or -1 for infinite retries (default:
+                                4)
     --http-timeout SECONDS      Timeout for HTTP connections (default: 30.0)
     --proxy URL                 Use the specified proxy
     --source-address IP         Client-side IP address to bind to