Add missing example language check.
Add missing example language check.

Summary:
Example output:

```
Each of the following components is missing example language:
components/FlexibleHeader/README.md:131
components/FlexibleHeader/README.md:159
components/FlexibleHeader/README.md:231
components/FlexibleHeader/README.md:322
components/FlexibleHeader/README.md:352
components/HeaderStackView/README.md:88
components/Ink/README.md:90
components/Ink/README.md:109
components/Ink/README.md:142
components/PageControl/README.md:121
components/PageControl/README.md:170
components/ShadowElevations/README.md:71
components/ShadowLayer/README.md:104
components/ShadowLayer/README.md:125
components/ShadowLayer/README.md:140
components/ShadowLayer/README.md:166
components/Slider/README.md:76
components/Switch/README.md:80
```

Reviewers: #mdc_ios_owners, ajsecord

Reviewed By: #mdc_ios_owners, ajsecord

Projects: #material_components_ios

Differential Revision: http://codereview.cc/D572

randallli/material-components-ios,sigito/material-components-ios,Eccelor/material-components-ios,chriscox/material-components-ios,randallli/material-components-ios,willlarche/material-components-ios,samnm/material-components-ios,sigito/material-components-ios,Eccelor/material-components-ios,chriscox/material-components-ios,samnm/material-components-ios,chriscox/material-components-ios,willlarche/material-components-ios,randallli/material-components-ios,Eccelor/material-components-ios,chriscox/material-components-ios,willlarche/material-components-ios,brianjmoore/material-components-ios,brianjmoore/material-components-ios,sigito/material-components-ios,samnm/material-components-ios,samnm/material-components-ios,randallli/material-components-ios,material-components/material-components-ios,Eccelor/material-components-ios,samnm/material-components-ios,brianjmoore/material-components-ios,material-components/material-components-ios,chriscox/material-components-ios,randallli/material-components-ios,willlarche/material-components-ios,material-components/material-components-ios,brianjmoore/material-components-ios,brianjmoore/material-components-ios,sigito/material-components-ios,material-components/material-components-ios,randallli/material-components-ios,Eccelor/material-components-ios,sigito/material-components-ios,willlarche/material-components-ios,material-components/material-components-ios