s390/hmcdrv: fix Kconfig "its" grammar

Use the possessive "its" instead of the contraction "it's"
where appropriate.
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Alexander Gordeev <agordeev@linux.ibm.com>
Cc: linux-s390@vger.kernel.org
Link: https://lore.kernel.org/r/20220715020010.12678-1-rdunlap@infradead.orgSigned-off-by: Alexander Gordeev <agordeev@linux.ibm.com>

drivers/s390/char/Kconfig

@@ -89,7 +89,7 @@ config HMC_DRV
 	  Management Console (HMC) drive CD/DVD-ROM. It is available as a
 	  module, called 'hmcdrv', and also as kernel built-in. There is one
 	  optional parameter for this module: cachesize=N, which modifies the
-	  transfer cache size from it's default value 0.5MB to N bytes. If N
+	  transfer cache size from its default value 0.5MB to N bytes. If N
 	  is zero, then no caching is performed.
 
 config SCLP_OFB