minor #5129 Fix to Twig asset function packageName argument (ockcyp)

This PR was submitted for the 2.5 branch but it was merged into the 2.6 branch instead (closes #5129).

Discussion
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Fix to Twig asset function packageName argument

Having no spaces surrounding ``|`` was causing the argument description to appear as
    **type**: ```string ``|`` null``` **default**: ```null```

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.5+
| Fixed tickets |

Commits
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b15c127 Fix to Twig asset function packageName argument

Author: wouterj

Diff for: reference/twig_reference.rst

@@ -103,7 +103,7 @@ asset
 ``path``
     **type**: ``string``
 ``packageName``
-    **type**: ``string``|``null`` **default**: ``null``
+    **type**: ``string`` | ``null`` **default**: ``null``
 ``absolute``
     **type**: ``boolean`` **default**: ``false``
 ``version``