Fixed incorrect theme variable used in theme file

[ArcaneAj]

docs(material): Code specified in documentation will not compile

The code as written in step 4 won't compile, since $theme is not defined.
Should pass $my-theme instead.

[google-cla]

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[ArcaneAj]

@googlebot I signed it!

[crisbeto]

The change looks good, but the CI is failing due to the formatting of your commit's title. It should be prefixed with docs:.

[ArcaneAj]

@crisbeto Is this correct?

[crisbeto]

It seems like you tried to rename the PR which won't do anything. You have to rename the commit which can't be done through the GitHub UI.

[devversion]

LGTM

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