Update block fixture README to use shorter friendlier version of comm…

…and (#22661)

* Use shorter friendlier version of command

* Update packages/e2e-tests/fixtures/blocks/README.md

Co-authored-by: Greg Ziółkowski <grzegorz@gziolo.pl>

Co-authored-by: Marcus Kazmierczak <marcus@mkaz.com>
Co-authored-by: Greg Ziółkowski <grzegorz@gziolo.pl>

packages/e2e-tests/fixtures/blocks/README.md

@@ -55,7 +55,7 @@ Next, to generate files (2) through (4) run the following command from the root
 project:
 
 ```sh
-GENERATE_MISSING_FIXTURES=y npm run test-unit test/integration/full-content/full-content.test.js
+npm run fixtures:regenerate test/integration/full-content/full-content.test.js
 ```
 
 When using this command, please be sure to manually verify that the
@@ -68,9 +68,9 @@ the attributes are serialized correctly.
 
 The process for updating fixtures for existing tests is similar to that for creating them:
 
-First delete files (2) through (4), then run the command to regenerate the files:
+Run the command to regenerate the files:
 ```sh
-GENERATE_MISSING_FIXTURES=y npm run test-unit test/integration/full-content/full-content.test.js
+npm run fixtures:regenerate test/integration/full-content/full-content.test.js
 ```
 
 After regenerating fixtures, check the diff (using git/github) to check that the changes were expected