update linkedin link in readme.md

[MaskedCarrot]

Explanation

The existing link to oppia's LinkedIn page shows profile not found on Linkedin.

Checklist

• The PR title starts with "Fix #bugnum: ", followed by a short, clear summary of the changes. (If this PR fixes part of an issue, prefix the title with "Fix part of #bugnum: ...".)
• The PR explanation includes the words "Fixes #bugnum: ..." (or "Fixes part of #bugnum" if the PR only partially fixes an issue).
• The PR follows the style guide.
• The PR does not contain any unnecessary auto-generated code from Android Studio.
• The PR is made from a branch that's not called "develop".
• The PR is made from a branch that is up-to-date with "develop".
• The PR's branch is based on "develop" and not on any other branch.
• The PR is assigned to an appropriate reviewer in both the Assignees and the Reviewers sections.

[MaskedCarrot]

@anandwana001 PTAL

[anandwana001]

Nice catch. Thanks @MaskedCarrot

[BenHenning]

I can't easily verify that this is correct now, but the old link does fail for me. This is indeed a nice catch, thanks @MaskedCarrot!