fix(prism.css): Remove background prop from .token.operator

[gdaffa]

Fix visual error within the code block by removing background property from .token.operator.

Before:

After:

For anyone wondering .token.operator class is defined on prism.css line 3 column 1479.

[TimWolla]

This bundled file should not be modified, because this will make updates harder. Instead the CSS should be overridden in some other way.

[gdaffa]

Oh im sorry i didnt know that, i also think that way but @shivammathur asked me to fix it on PR #1838 so i think its fine.

[shivammathur]

@gdaffa Please use styles/code-syntax.css instead to override this.