Fix errors in SourcePluginBase doc block

Problem/Motivation

The class doc block in Drupal\migrate\Plugin\migrate\source\SourcePluginBase includes these lines:

 * - The row needs an update.
 *   - Rows can be marked by custom or contrib modules using the
 *     \Drupal\migrate\Plugin\MigrateIdMapInterface::prepareUpdate() os
 *     \Drupal\migrate\Plugin\MigrateIdMapInterface::setUpdate()
 *     methods.

The word "os" should be "or".

Further down, there is this code example:

 * @code
 * source:
 *   plugin: some_source_plugin_name
 *   constants:
 *     - foo: bar
 * process:
 *   baz: constants/bar
 * @endcode
 *
 * In this example, the constant 'foo' is defined with a value of 'bar'. It is
 * later used in the process pipeline to set the value of the field baz.

There are two problems here:

• The line * - foo: bar should be just * foo: bar (no -). Thanks to @abramley for catching this mistake.
• The line * baz: constants/bar should be * baz: constants/foo.

Proposed resolution

Make the requested changes.

Remaining tasks

User interface changes

None

Introduced terminology

None

API changes

None

Data model changes

None

Release notes snippet

N/A