Material-ui: [Docs] Improve grammar in docs

[DDDDDanica]

Propose few fixes in snackbar.md, selects.md, dialogs.md and progress.md

[oliviertassinari]

:)

[DDDDDanica]

• [x] snackbars.md: title Positioned => Should be Positioned Snackbar

• [x] snackbars.md: title Simple => Should be Simple Snackbar

• [x] snackbard.md: 'Snackbars provide brief messages about app processes through a message - typically at the bottom of the screen' => missing a period at the end of the sentence

• [x] selects.md: ’Controlled open Select' => open should be capitalised here

• [x] selects.md: 'Once it's styled, you can either use it directly as a text field or provide it to the select <0>input property to have a select field.' => If I understand it right, in this sentence: 'provide it to the select input property to have a Select field.' , while 'Select' should be the name of the component, right? It means we should make it capitalized or have some code wrapper around it :)

• [x] dialogs.md: 'Here is example of customizing the component' => should be an example

• [x] progress.md: 'When it's not possible, you can leverage the <0>disableShrink property to mitigate the issue.
  See https://github.com/mui-org/material-ui/issues/10327"

• A period is missing at the end of the sentence

• There should be a hyper link around https://github.com/mui-org/material-ui/issues/10327

• [x] buttons.md: at the end of Customized buttons section and before Note: Creating the Button components is ne... there's an empty string captured in the doc
  
  .

[joshwooding]

’Controlled open Select' => open should be capitalised here

Since open is a prop this is probably fine. Maybe we should use ` to denote this?

[oliviertassinari]

@DDDDDanica Could you detail your objective with this issue? It seems to report inconsistencies with our documentation in English but I don't understand, what are the checkboxes for? Thanks

[DDDDDanica]

@oliviertassinari Sorry for the confusion, so the object for this issue is to change some grammar problems in document. Checkboxes are for @mbrookes to keep track of the issue I raised. :)