otasigcheck: Fix the sed statement for multi-digit indices

It appears that some versions of sed do not work with the + symbol.
Instead of checking for one or more digits, check for a digit,
followed by zero or more digits.

Change-Id: I064df6a2bac4a634a3684ac1a5289dca1f4ba29c
(cherry picked from commit 8cc3992a9f24337930d4da3e14a9025f7af62055)