Change lower-case and upper-case to match recommendations in python#88211

Issue python#88211 recommends canonicalizing instances of upper-case to
uppercase and lower-case to lowercase. This commit addresses
outstanding instances present in Doc/library/impamlib.rst.

Author: tbwolfe

Doc/library/imaplib.rst

@@ -187,7 +187,7 @@ IMAP4 Objects
 -------------
 
 All IMAP4rev1 commands are represented by methods of the same name, either
-upper-case or lower-case.
+uppercase or lowercase.
 
 All arguments to commands are converted to strings, except for ``AUTHENTICATE``,
 and the last argument to ``APPEND`` which is passed as an IMAP4 literal.  If