README: prevent a python warning

slapos/recipe/cmmi/README.rst

@@ -875,7 +875,7 @@ can display "prefix" value in the stdout.
     ... recipe = slapos.recipe.cmmi
     ... url = file://%s/package-0.0.0.tar.gz
     ... configure-command = ./configure
-    ... pre-install = sed -i -e "s/installing package/installing package at \$\$prefix /g" Makefile
+    ... pre-install = sed -i -e "s/installing package/installing package at \\$\\$prefix /g" Makefile
     ... """ % src)
 
     >>> print(system(buildout))
@@ -901,7 +901,7 @@ replaced with the recipe final prefix.
     ... url = file://%s/package-0.0.0.tar.gz
     ... configure-command = ./configure
     ... make-targets = install-lib prefix=%%(prefix)s
-    ... pre-install = sed -i -e "s/installing package/installing package at \$\$prefix /g" Makefile
+    ... pre-install = sed -i -e "s/installing package/installing package at \\$\\$prefix /g" Makefile
     ... """ % src)
 
     >>> print(system(buildout))