Fix --metadata doc usage (#1112)

Thanks, @jefftriplett.

docs/changelog.rst

@@ -453,7 +453,7 @@ You can now create :ref:`custom pages <custom_pages>` within your Datasette inst
 :ref:`settings_dir` (`#731 <https://github.com/simonw/datasette/issues/731>`__) allows you to define a custom Datasette instance as a directory. So instead of running the following::
 
     $ datasette one.db two.db \
-      --metadata.json \
+      --metadata=metadata.json \
       --template-dir=templates/ \
       --plugins-dir=plugins \
       --static css:css

docs/settings.rst

@@ -23,7 +23,7 @@ Configuration directory mode
 Normally you configure Datasette using command-line options. For a Datasette instance with custom templates, custom plugins, a static directory and several databases this can get quite verbose::
 
     $ datasette one.db two.db \
-        --metadata.json \
+        --metadata=metadata.json \
         --template-dir=templates/ \
         --plugins-dir=plugins \
         --static css:css