docs: correct trailer key_value_separator description

The description for `key_value_separator` incorrectly states that this
separator is inserted between trailer lines, which appears likely to
have been incorrectly copied from `separator` when this option was
added.

Update the description to correctly indicate that it is a separator that
appears between the key and the value of each trailer.

Documentation/pretty-formats.txt

@@ -329,9 +329,9 @@ multiple times, the last occurrence wins.
    `%(trailers:only,unfold=true)` unfolds and shows all trailer lines.
 ** 'keyonly[=<bool>]': only show the key part of the trailer.
 ** 'valueonly[=<bool>]': only show the value part of the trailer.
-** 'key_value_separator=<sep>': specify a separator inserted between
-   trailer lines. When this option is not given each trailer key-value
-   pair is separated by ": ". Otherwise it shares the same semantics
+** 'key_value_separator=<sep>': specify a separator inserted between each
+   trailer's key and value. When this option is not given each trailer
+   key-value pair is separated by ": ". Otherwise it shares the same semantics
    as 'separator=<sep>' above.
 
 NOTE: Some placeholders may depend on other options given to the