Update README

Author: mbostock

Diff for: README.md

@@ -651,7 +651,7 @@ All marks support the following optional channels:
 
 The **fill**, **fillOpacity**, **stroke**, **strokeWidth**, **strokeOpacity**, and **opacity** options can be specified as either channels or constants. When the fill or stroke is specified as a function or array, it is interpreted as a channel; when the fill or stroke is specified as a string, it is interpreted as a constant if a valid CSS color and otherwise it is interpreted as a column name for a channel. Similarly when the fill opacity, stroke opacity, object opacity, stroke width, or radius is specified as a number, it is interpreted as a constant; otherwise it is interpreted as a channel.
 
-The **title**, **href**, and **ariaLabel** options can *only* be specified as channels. When these options are specified as strings, the strings refer to the names of columns in the associated data. If you’d like every instance of a particular mark to have the same value, specify the option as a function that returns the desired value, *e.g.* `() => "Hello, world!"`.
+The **title**, **href**, and **ariaLabel** options can *only* be specified as channels. When these options are specified as a string, the string refers to the name of a column in the mark’s associated data. If you’d like every instance of a particular mark to have the same value, instead specify the option as a function that returns the desired value, *e.g.* `() => "Hello, world!"`.
 
 The rectangular marks ([bar](#bar), [cell](#cell), and [rect](#rect)) support insets and rounded corner constant options: