fix README: arrow default strokeWidth=1 (#922)

reported by @yurivish

Author: Fil

README.md

@@ -837,7 +837,7 @@ The following channels are required:
 
 For vertical or horizontal arrows, the **x** option can be specified as shorthand for **x1** and **x2**, and the **y** option can be specified as shorthand for **y1** and **y2**, respectively.
 
-The arrow mark supports the [standard mark options](#marks). The **stroke** defaults to currentColor. The **fill** defaults to none. The **strokeWidth** and **strokeMiterlimit** default to one. The following additional options are supported:
+The arrow mark supports the [standard mark options](#marks). The **stroke** defaults to currentColor. The **fill** defaults to none. The **strokeWidth** defaults to 1.5, and the **strokeMiterlimit** defaults to one. The following additional options are supported:
 
 * **bend** - the bend angle, in degrees; defaults to zero
 * **headAngle** - the arrowhead angle, in degrees; defaults to 22.5°