Fix typo in command for checking single doc file

The recommended command line option for running the doc check on a single documentation file contained a typo.

h/t to @nik9000 for finding the typo.

docs/README.asciidoc

@@ -6,7 +6,7 @@ See: https://github.com/elastic/docs
 Snippets marked with `// CONSOLE` are automatically annotated with "VIEW IN
 SENSE" in the documentation and are automatically tested by the command
 `gradle :docs:check`. To test just the docs from a single page, use e.g.
-`gradle :docs:check -Dtest.method=*rollover*`.
+`gradle :docs:check -Dtests.method=*rollover*`.
 
 By default `// CONSOLE` snippet runs as its own isolated
 test. You can manipulate the test execution in the following ways: