Fix table of contents numbering and 'Deep Leaning' typo in README

[Manya Sharma (ManyaS-Git)]

Summary

Two small fixes in \README.md:

1. Table of contents numbering: The \Contributing\ entry is numbered \6.\ but follows entries 1-4, leaving a gap (the \5.\ was lost). Rendered markdown renumbers automatically, but the raw file is incorrect and confusing for contributors reading the source.
2. Typo: \Deep Leaning\ -> \Deep Learning\ in the AI200 table (\Train Distributed Deep Leaning Model). All other rows in the same table already use \Deep Learning.