minor symfony#9690 Incompatible ts-loader version (nicholasruunu)

This PR was submitted for the 4.0 branch but it was merged into the 3.4 branch instead (closes symfony#9690).

Discussion
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Incompatible ts-loader version

The ts-loader major version must map to the webpack major version:
TypeStrong/ts-loader#748

Since symfony/encore depends on webpack 3, installing latest will produce the error above.

Commits
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8f2d2f1 Incompatible ts-loader version

Author: javiereguiluz

frontend/encore/typescript.rst

@@ -5,7 +5,7 @@ Want to use `TypeScript`_? No problem! First, install the dependencies:
 
 .. code-block:: terminal
 
-    $ yarn add --dev typescript ts-loader
+    $ yarn add --dev typescript ts-loader@^3.0
 
 Then, activate the ``ts-loader`` in ``webpack.config.js``: