Update docs/html/development/architecture/package-finding.rst
Finding and choosing files (``index`` and ``PackageFinder``)
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-The ``pip._internal.index`` sub-package in pip responsible for deciding
+The ``pip._internal.index`` sub-package in pip is responsible for deciding
what file to download and from where, given a requirement for a project. The
package's functionality is largely exposed through and coordinated by the
package's ``PackageFinder`` class.
