Adding a note on dvc metrics diff --targets with revisions usage

[imhardikj]

As per suggestion by @jorgeorpinel in #1549 (comment)

[jorgeorpinel]

Almost. What about plots diff though, probably same deal? Thanks @imhardikj

[jorgeorpinel]

Let's just avoid this sentence and change "may" for "must" or "should" in the previous one, please. Also, "the" is missing again 😞

Suggested change

	Without `--`, this statement will show an error as argument parser confuses
	`revisions` as arguments for `--targets` option. Alternatively, you can also
	run above statement as:
	Alternatively, you can also run the above statement as:

Same for plots diff, tnx

[jorgeorpinel]

Thanks @imhardikj