selftests: pmtu: fix init mtu value in description

There is no a_r3, a_r4 in the testing topology.
It should be b_r1, b_r2. Also b_r1 mtu is 1400 and b_r2 mtu is 1500.

Fixes: e44e428f59 ("selftests: pmtu: add basic IPv4 and IPv6 PMTU tests")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Acked-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

tools/testing/selftests/net/pmtu.sh

@@ -11,9 +11,9 @@
 #	R1 and R2 (also implemented with namespaces), with different MTUs:
 #
 #	  segment a_r1    segment b_r1		a_r1: 2000
-#	.--------------R1--------------.	a_r2: 1500
-#	A                               B	a_r3: 2000
-#	'--------------R2--------------'	a_r4: 1400
+#	.--------------R1--------------.	b_r1: 1400
+#	A                               B	a_r2: 2000
+#	'--------------R2--------------'	b_r2: 1500
 #	  segment a_r2    segment b_r2
 #
 #	Check that PMTU exceptions with the correct PMTU are created. Then