fix erroneous docstring of sh.Exec() (#452)

sh.Exec() uses the passed in stderr io.Writer to write the stderr output
not "piping its stderr to mage's stderr".

sh/cmd.go

@@ -89,14 +89,14 @@ func OutputWith(env map[string]string, cmd string, args ...string) (string, erro
 	return strings.TrimSuffix(buf.String(), "\n"), err
 }
 
-// Exec executes the command, piping its stderr to mage's stderr and
-// piping its stdout to the given writer. If the command fails, it will return
-// an error that, if returned from a target or mg.Deps call, will cause mage to
-// exit with the same code as the command failed with.  Env is a list of
-// environment variables to set when running the command, these override the
-// current environment variables set (which are also passed to the command). cmd
-// and args may include references to environment variables in $FOO format, in
-// which case these will be expanded before the command is run.
+// Exec executes the command, piping its stdout and stderr to the given
+// writers. If the command fails, it will return an error that, if returned
+// from a target or mg.Deps call, will cause mage to exit with the same code as
+// the command failed with. Env is a list of environment variables to set when
+// running the command, these override the current environment variables set
+// (which are also passed to the command). cmd and args may include references
+// to environment variables in $FOO format, in which case these will be
+// expanded before the command is run.
 //
 // Ran reports if the command ran (rather than was not found or not executable).
 // Code reports the exit code the command returned if it ran. If err == nil, ran