Update Figure 1 to say "Basic" rather than "Baseline"

[benfrancis]

PR #265 will rename the "HTTP Baseline Profile" to the "HTTP Basic Profile".

The image from Figure 1 and its caption will need to be updated to reflect this change.

(Note that the whole of the Abstract & Introduction sections are going to be reviewed in #115 and it's possible the figure will be removed altogether).

[mlagally]

See #309 , I believe we need an overview figure.