Fixed Broken links at various Documentation files

[chetkhatri]

What were proposed in this Pull Request ?

Broken links has been fixed for following documentations pages.

• R-package/README.md
• README.md
• docs/api/scala/index.md
• docs/tutorials/index.md
• docs/tutorials/scala/char_lstm.md
• docs/tutorials/scala/mnist.md
• docs/tutorials/scala/mxnet_scala_on_intellij.md

@madjam Can you please review this Pull Request and merge if looks good.

[chetkhatri]

@mli Ideally I must be added to contributor list, but you have n't provided argument as a author=chetkhatri

[mli]

you can file another PR to add your name into https://github.com/dmlc/mxnet/blob/master/CONTRIBUTORS.md

[chetkhatri]

@mli Yeah I can but earlier commits would not be visible at contributors section, We faced same issue at Apache HBase contribution. would you mind to do git commit --author=. i.e the guideline is to use git am --signoff 👍