curve is described by not defined by (#24)

Addresses #16 using @mattsimpson3 's proposal. Merging early as a minor editorial change.

Author: nigelmegitt

index.html

@@ -202,7 +202,7 @@ <h3>Define audio mixing instructions: <dfn data-lt="PS4">Process Step 4</dfn>. <
                 This is typically a single value that applies to each description.</li>
             <li>Select the amount by which to lower the main programme audio
                 prior to mixing in the description audio.
-                This is typically defined as a curve defined by a set of moments in time
+                This is typically defined as a curve described by a set of moments in time
                 and fade levels to apply,
                 with an interpolation algorithm to vary the levels between each moment in time.</li>
           </ul>