trace2: correct typo in technical documentation

an apparent typo for the environment variable was included with 81567caf87
("trace2: update docs to describe system/global config settings",
2019-04-15), and was missed when renamed variables by e4b75d6a1d
("trace2: rename environment variables to GIT_TRACE2*", 2019-05-19)

Signed-off-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/technical/api-trace2.txt

@@ -35,7 +35,7 @@ Format details are given in a later section.
 === The Normal Format Target
 
 The normal format target is a tradition printf format and similar
-to GIT_TRACE format.  This format is enabled with the `GIT_TR`
+to GIT_TRACE format.  This format is enabled with the `GIT_TRACE2`
 environment variable or the `trace2.normalTarget` system or global
 config setting.