fall-back -> fallback find-doc-nit addition

Ensure the same term is used for fallback

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/13862)

util/find-doc-nits

@@ -609,6 +609,7 @@ my %preferred_words = (
     'bitmask'       => 'bit mask',
     'builtin'       => 'built-in',
    #'epoch'         => 'Epoch', # handled specially, below
+    'fall-back'     => 'fallback',
     'file name'     => 'filename',
     'file system'   => 'filesystem',
     'host name'     => 'hostname',