docs: parse-headers.rst: remove uneeded parenthesis

As pointed by Randy, the parenthesis there is not needed and it
violates the document coding style.

Reported-by: Randy Dunlap <[e-mail]>
Closes: https://lore.kernel.org/linux-doc/[e-mail]/
Signed-off-by: Mauro Carvalho Chehab <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>
Message-ID: <e5de9f[e-mail]>

Documentation/doc-guide/parse-headers.rst

···
 It is meant to allow having more comprehensive documentation, where
 uAPI headers will create cross-reference links to the code.
 
-The output is written at the (``FILE_OUT``).
+The output is written at the ``FILE_OUT``.
 
 The ``FILE_RULES`` may contain contain three types of statements:
 **ignore**, **replace** and **namespace**.