dt-bindings: mmc: Fix node name in an example

The $nodename allows only "mmc@*" whereas the example node is named
"sdhci".

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Link: https://lore.kernel.org/r/20200317093922.20785-19-lkundrak@v3.sk


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>