tcl: fix typo and spelling

Identified by checkpatch script from Linux kernel v5.7-rc1 using
the command
	find tcl/ -type f -exec ./tools/scripts/checkpatch.pl \
	-q --types TYPO_SPELLING --strict -f {} \;

Change-Id: I7b523f0ab5ec047ff167742a44c29984ac672cf4
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5615
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Tested-by: jenkins

tcl/target/lpc2900.cfg

@@ -14,7 +14,7 @@ if { [info exists CPUTAPID] } {
 if { [info exists HAS_ETB] } {
 } else {
     # Set default (no ETB).
-    # Show a warning, because this should have been configured explicitely.
+    # Show a warning, because this should have been configured explicitly.
     set HAS_ETB 0
     # TODO: warning?
 }