docs($httpParamSerializer): Remove quote

[avianbc]

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
docs update

What is the current behavior? (You can also link to an open issue here)
Has an unnecessary quote. I double checked by URL decoding the string.

What is the new behavior (if this is a feature change)?
Removed quote

Does this PR introduce a breaking change?
No

Please check if the PR fulfills these requirements

• [ X] The commit message follows our guidelines: https://github.com/angular/angular.js/blob/master/CONTRIBUTING.md#commit-message-format
• [ X] Tests for the changes have been added (for bug fixes / features)
• [ X] Docs have been added / updated (for bug fixes / features)

Other information:

Remove superfluous quote from docs. Change done on the fly.

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[avianbc]

I signed it!

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