fix(v2): make proper h1 font size on mobiles

[lex111]

Motivation

According to Infima styles https://github.com/facebookincubator/infima/blob/master/packages/core/styles/content/markdown.css#L36-L41

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

Before	After

Related PRs

(If this PR adds or changes functionality, please take some time to update the docs at https://github.com/facebook/docusaurus, and link to your PR here.)

[docusaurus-bot]

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[lex111]

Test fails because this:

 FAIL  packages/**********-1.x/lib/server/__tests__/utils.test.js (78.747s)
  server utils
    ✕ minify css (5634ms)
    ✓ autoprefix css (68ms)
    ✓ getLanguage (4ms)
    ✓ getSubdir (5ms)

  ● server utils › minify css
>     : Timeout - Async callback was not invoked within the 5000ms timeout specified by jest.setTimeout.Timeout - Async callback was not invoked within the 5000ms timeout specified by jest.setTimeout.Error:

Curious, should we increase the timeout for build of v1?

[yangshun]

Let's see if the timeout error still persists.