build: correct minor toolchain documentation error

Default values for toolchain tools are instead provided by the main
toolchain makefile, rather than a parent makefile. This was an oversight
from a previous version of the original toolchain refactor patch.

Change-Id: I75752ed7874b36e1c679d94292a2664e234c484b
Signed-off-by: Chris Kay <chris.kay@arm.com>
diff --git a/make_helpers/toolchain.mk b/make_helpers/toolchain.mk
index 505630e..7255509 100644
--- a/make_helpers/toolchain.mk
+++ b/make_helpers/toolchain.mk
@@ -158,7 +158,7 @@
 #
 # Toolchains can specify a default path to any given tool with a tool class.
 # These values are used in the absence of user-specified values, and are
-# configured by the parent Makefile using variables of the form:
+# configured by the makefile for each toolchain using variables of the form:
 #
 #   - $(toolchain)-$(tool-class)-default
 #