cmd/gc, cmd/ld: update doc.go for -race

R=golang-dev, dave, rsc
CC=golang-dev
https://golang.org/cl/7066052

src/cmd/gc/doc.go

@@ -54,7 +54,7 @@ Flags:
 		disallow importing packages not marked as safe
 	-V
 		print the compiler version
-	-b
+	-race
 		compile with race detection enabled
 
 There are also a number of debugging flags; run the command with no arguments

src/cmd/ld/doc.go

@@ -56,7 +56,7 @@ Options new in this version:
 		Set the value of an otherwise uninitialized string variable.
 		The symbol name should be of the form importpath.name,
 		as displayed in the symbol table printed by "go tool nm".
-	-b
+	-race
 		Link with race detection libraries.
 	-B value
 		Add a NT_GNU_BUILD_ID note when using ELF.  The value