Docker build instructions - suggest small improvement

[Daksol]

In your instructions for building a new Image with the macros module included, it might be useful to mention that there is an essential period character at the end of that line. The period char is there in the notes, but easy to miss!!

docker build -t squidfunk/mkdocs-material .

Good trap for the unwary otherwise - the error message ERROR: "docker buildx build" requires exactly 1 argument. is pretty unhelpful if you leave the period character out.

[michaelherger]

Would you mind creating a PR?...

[Daksol]

I will do so.