Documentation/git.txt: clarify that GIT_TRACE=/path appends

The current wording of the description of GIT_TRACE=/path/to/file
("... will try to write the trace messages into it") might be
misunderstood as "overwriting"; at least I interpreted it that way on
a cursory first read.

State it more explicitly that the trace messages are appended.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git.txt

@@ -599,8 +599,8 @@ trace messages into this file descriptor.
 +
 Alternatively, if the variable is set to an absolute path
 (starting with a '/' character), Git will interpret this
-as a file path and will try to write the trace messages
+as a file path and will try to append the trace messages
-into it.
+to it.
 +
 Unsetting the variable, or setting it to empty, "0" or
 "false" (case insensitive) disables trace messages.