Fix typo

[SVN r40473]

Jamroot

@@ -442,7 +442,7 @@ alias build_all : libs/$(libraries)/build ;
 
 # This rule should be called from libraries' Jamfiles and will
 # create two targets, "install" and "stage", that will install
-# or stage that library. The --prefix option is respected, by
+# or stage that library. The --prefix option is respected, but
 # --with and --without options, naturally, are ignored.
 #
 # - libraries -- list of library targets to install.