Fix default input channel name in mxnet README

[icywang86rui]

• Change the default channel name from 'train' to 'training'

Issue #332, if available:

Description of changes:

Merge Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your pull request.

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works (if appropriate)
• I have updated the changelog with a description of my changes (if appropriate)
• I have updated any necessary documentation (if appropriate)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[yangaws]

Thanks!

[codecov-io]

Codecov Report

Merging #333 into master will increase coverage by 0.16%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #333      +/-   ##
==========================================
+ Coverage   92.86%   93.03%   +0.16%     
==========================================
  Files          51       51              
  Lines        3560     3560              
==========================================
+ Hits         3306     3312       +6     
+ Misses        254      248       -6

Impacted Files	Coverage Δ
src/sagemaker/local/image.py	88.85% <0%> (+1.75%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update f4021f9...86c468e. Read the comment docs.