doc: fix typo in docbuild.rst
Fix a typo in the `docbuild.rst` file.
@@ -179,7 +179,7 @@ another ``make html`` and the output layout and style is changed. The
sphinx build system creates document cache information that attempts to
expedite documentation rebuilds, but occasionally can cause an unexpected error or
warning to be generated. Doing a ``make clean`` to create a clean
-generation environent and a ``make html`` again generally cleans this up.
+generation environment and a ``make html`` again generally cleans this up.
The ``read-the-docs`` theme is installed as part of the
``requirements.txt`` list above. Tweaks to the standard
