[doc] Fix typo: impl {MyType==>Counter}.
[doc] Fix typo: impl {MyType==>Counter}.

Signed-off-by: Dan Nussbaum <86aa56cfef221ac21e80998bdebfb5a7b6754cdd@google.com>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan