Don't mention inexistend outLen Parameter in (*Program).Test godoc

The outLen parameter to (*Program).Test was removed in commit
e45ae4a ("detect kernel support for BPF_PROG_TEST_RUN and work
around unsafe API"). Remove it from the godoc comment as well.

Signed-off-by: Tobias Klauser <tobias@cilium.io>

prog.go

@@ -659,7 +659,7 @@ type RunOptions struct {
 }
 
 // Test runs the Program in the kernel with the given input and returns the
-// value returned by the eBPF program. outLen may be zero.
+// value returned by the eBPF program.
 //
 // Note: the kernel expects at least 14 bytes input for an ethernet header for
 // XDP and SKB programs.