[3.14] gh-148814: Fix an issue in Emscripten README (GH-148752) (#148815)

Correct the description of the default state of test module compilation.
(cherry picked from commit d206d42)

Co-authored-by: Stan Ulbrych <stan@python.org>

Author: miss-islington

Platforms/emscripten/README.md

@@ -243,8 +243,8 @@ await createEmscriptenModule({
   are not shipped. All other modules are bundled as pre-compiled
   ``pyc`` files.
 - In-memory file system (MEMFS) is not persistent and limited.
-- Test modules are disabled by default. Use ``--enable-test-modules`` build
-  test modules like ``_testcapi``.
+- Test modules are built by default. Use ``--disable-test-modules`` to disable
+  building test modules like ``_testcapi``.
 
 ## WASI (wasm32-wasi)