Correct speliing of --build-dir in --help output

[SVN r42997]

Jamroot

@@ -53,7 +53,7 @@
 #                               complete - Attempts to build all possible
 #                               variations.
 #
-#   --builddir=DIR          Build in this location instead of building
+#   --build-dir=DIR         Build in this location instead of building
 #                           within the distribution tree. Recommended!
 #
 #   --toolset=toolset       Indicates the toolset to build with.