Update conformance codelab instructions

[roger2hk]

I got unexpected result when running the codelab instructions.

[ec2-user@ip-172-31-24-203 ~]$ # Add 3 entries in parallel, and wait for all requests to complete
curl -d 'one!' -H "Content-Type: application/data" -X POST ${WRITE_URL}add &
curl -d 'two!' -H "Content-Type: application/data" -X POST ${WRITE_URL}add &
curl -d 'three!' -H "Content-Type: application/data" -X POST ${WRITE_URL}add &
wait

# Check that the checkpoint is of the correct size
curl -s ${READ_URL}checkpoint

# Look at the leaves. Piping into xxd to reveal the leaf sizes.
curl -s ${READ_URL}tile/entries/000.p/3 | xxd
[1] 32012
[2] 32013
[3] 32014
012[1]   Done                    curl -d 'one!' -H "Content-Type: application/data" -X POST ${WRITE_URL}add
[2]-  Done                    curl -d 'two!' -H "Content-Type: application/data" -X POST ${WRITE_URL}add
[3]+  Done                    curl -d 'three!' -H "Content-Type: application/data" -X POST ${WRITE_URL}add
codelab-886436940283-trillian-tessera
0
47DEQpj8HBSa+/TImW+5JCeuQeRkm5NMpJWZG3hSuFU=

— codelab-886436940283-trillian-tessera UYSAAgHen0fA6IoEfaXDql9G9ruSXHowUougX9U69Mf+RHo84v1l8IbWl94RF77R95titHud616qKmjyj9gr1Q+mGww=
00000000: 3c3f 786d 6c20 7665 7273 696f 6e3d 2231  <?xml version="1
00000010: 2e30 2220 656e 636f 6469 6e67 3d22 5554  .0" encoding="UT
00000020: 462d 3822 3f3e 0a3c 4572 726f 723e 3c43  F-8"?>.<Error><C
00000030: 6f64 653e 4163 6365 7373 4465 6e69 6564  ode>AccessDenied
00000040: 3c2f 436f 6465 3e3c 4d65 7373 6167 653e  </Code><Message>
00000050: 4163 6365 7373 2044 656e 6965 643c 2f4d  Access Denied</M
00000060: 6573 7361 6765 3e3c 5265 7175 6573 7449  essage><RequestI
00000070: 643e 3231 4745 314d 4656 4759 3151 4341  d>21GE1MFVGY1QCA
00000080: 544d 3c2f 5265 7175 6573 7449 643e 3c48  TM</RequestId><H
00000090: 6f73 7449 643e 556c 4374 4c41 6345 4667  ostId>UlCtLAcEFg
000000a0: 7034 3266 3630 3359 7232 412b 3049 4741  p42f603Yr2A+0IGA
000000b0: 797a 6335 6b55 4553 7842 357a 6a48 7132  yzc5kUESxB5zjHq2
000000c0: 6855 4671 4c34 4262 4147 7934 4f4c 6538  hUFqL4BbAGy4OLe8
000000d0: 7266 4f32 2b4b 5150 7765 7663 3877 6a50  rfO2+KQPwevc8wjP
000000e0: 343d 3c2f 486f 7374 4964 3e3c 2f45 7272  4=</HostId></Err
000000f0: 6f72 3e                                  or>