Man-downcase-section-letters-flag is a variable defined in `man.el'.
Its value is

Non-nil means letters in sections are converted to lower case.
Some Un*x man commands can't handle uppercase letters in sections, for
example "man 2V chmod", but they are often displayed in the manpage
with the upper case letter. When this variable is t, the section
letter (e.g., "2V") is converted to lowercase (e.g., "2v") before
being sent to the man background process.

You can customize this variable.