updating_clang.md: Add win7-rel to trybot list

It's no longer run by the CQ by default
(see "[chromium-dev] [PSA] win7-rel become optional in CQ").

TBR=thakis

Bug: none
Change-Id: I2993390c12e0445b58ebb6868c512ee65f129c6b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1758063Reviewed-by: Hans Wennborg <hans@chromium.org>
Commit-Queue: Hans Wennborg <hans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#687596}

docs/updating_clang.md

@@ -46,7 +46,7 @@ An archive of all packages built so far is at https://is.gd/chromeclang
       -b linux_chromium_chromeos_asan_rel_ng -b linux_chromium_msan_rel_ng \
       -b linux_chromium_chromeos_msan_rel_ng -b linux-chromeos-dbg \
       -b win-asan -b chromeos-amd64-generic-cfi-thin-lto-rel \
-      -b linux_chromium_compile_dbg_32_ng &&
+      -b linux_chromium_compile_dbg_32_ng -b win7-rel &&
     git cl try -B luci.chrome.try -b iphone-device -b ipad-device
     ```