make backslashes visible in docs for dict_add and dict_set (#1372)

In order to escape colons in the keys and values that you provide to dict_add and dict_set, you must precede them with backslashes.  The raw README.md file has the backslashes, but they are not visibile when rendered.  We here double these backslashes so that they'll be visible when rendered.
diff --git a/buildozer/README.md b/buildozer/README.md
index 9d259e4..ab01311 100644
--- a/buildozer/README.md
+++ b/buildozer/README.md
@@ -142,10 +142,10 @@
     rules. If it exists in the `to_rule`, no action is taken.
   * `dict_add <attr> <(key:value)(s)>`:  Sets the value of a key for the dict
     attribute `attr`. If the key was already present, it will _not_ be overwritten.
-    Colon characters can be included in key and value escaped as "\:".
+    Colon characters can be included in key and value escaped as "\\:".
   * `dict_set <attr> <(key:value)(s)>`:  Sets the value of a key for the dict
     attribute `attr`. If the key was already present, its old value is replaced.
-    Colon characters can be included in key and value escaped as "\:".
+    Colon characters can be included in key and value escaped as "\\:".
   * `dict_remove <attr> <key(s)>`:  Deletes the key for the dict attribute `attr`.
   * `dict_replace_if_equal <attr> <key> <old_value> <new_value>`: Replaces
     `old_value` with `new_value` for key `key` in dictionary attribute `attr`.