docs: parse-headers.rst: Fix a typo

Sometimes, it is useful to include header files and C example codes in

order to describe the userspace API and to generate cross-references

between the code and the documentation. Adding cross-references for

userspace API files has an additional vantage: Sphinx will generate warnings

userspace API files has an additional advantage: Sphinx will generate warnings

if a symbol is not found at the documentation. That helps to keep the

uAPI documentation in sync with the Kernel changes.

The :ref:`parse_headers.py <parse_headers>` provides a way to generate such