minor symfony#5221 Update Uglifyjs.rst (assoum891)

This PR was submitted for the 2.6 branch but it was merged into the 2.3 branch instead (closes symfony#5221).

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Update Uglifyjs.rst

a spelling correction

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0f95bb5 Update Uglifyjs.rst

Author: wouterj

cookbook/assetic/uglifyjs.rst

@@ -40,7 +40,7 @@ Local Installation
 ~~~~~~~~~~~~~~~~~~
 
 It's also possible to install UglifyJS inside your project only, which is useful
-when your project requires an specific UglifyJS version. To do this, install it
+when your project requires a specific UglifyJS version. To do this, install it
 without the ``-g`` option and specify the path where to put the module:
 
 .. code-block:: bash