The ornamental design for an adjustable crank arm, as shown and described.
FIG. 1 is an isometric view of an adjustable crank arm;
FIG. 2 is a right side view thereof;
FIG. 3 is a left side view thereof;
FIG. 4 is a back view thereof;
FIG. 5 is a front view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The broken line showing of the adjustable crank arm is for the purpose of illustrating portions of the article and forms no part of the claimed design.