DOC: Remove duplicate 'in' from contributing.rst (pandas-dev#18040) (pandas-dev#18076)

(cherry picked from commit 21f9e1a)

Author: mattayes

doc/source/contributing.rst

@@ -331,7 +331,7 @@ The utility script ``scripts/api_rst_coverage.py`` can be used to compare
 the list of methods documented in ``doc/source/api.rst`` (which is used to generate
 the `API Reference <http://pandas.pydata.org/pandas-docs/stable/api.html>`_ page)
 and the actual public methods.
-This will identify methods documented in in ``doc/source/api.rst`` that are not actually
+This will identify methods documented in ``doc/source/api.rst`` that are not actually
 class methods, and existing methods that are not documented in ``doc/source/api.rst``.