capitalization: Add 'I understand' to the IGNORED_PHRASES list.

String 'Here are a few messages I understand:'(next commit) was failing
./tools/check-capitalization check because of the capital I. I added
'I understand' to the IGNORED_PHRASES list in tools/lib/capitalization.py.

Adding "I" was working as well but didn't seem to me as a very great fix.
Strangely enough, adding " I " to the list made the test fail again
(With a lot of failed strings this time) as mentioned in the following
CZO thread.

Relevent CZO chat -
https://chat.zulip.org/#narrow/stream/49-development-help/topic/capitalization.20confusion.2E

tools/lib/capitalization.py

@@ -72,6 +72,7 @@
     r".zuliprc",
     r"__\w+\.\w+__",
     # Things using "I"
+    r"I understand",
     r"I say",
     r"I want",
     r"I'm",