[css-exclusions][editorial] typo

Small typo I found while reading the spec:

  # Exclusion element
  An block-level element which is not a float [...]

should be

  # Exclusion element
  A block-level element which is not a float [...]



By the way, I think mentioning the title in the sentence would improve clarity:

  An <b>exclusion element</b> is a block-level element which...