rhashtable: Improve grammar

Change "a" to "an" according to the usual rules, fix an "if" that
was mistyped as "in", improve grammar in "considerable slow" ->
"considerably slower".

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/20240329-misc-rhashtable-v1-1-5862383ff798@gmx.net


Signed-off-by: Jakub Kicinski <kuba@kernel.org>