org-agenda.el (org-agenda-custom-commands-contexts): Fix docstring

* org-agenda.el (org-agenda-custom-commands-contexts): Fix docstring.

lisp/org-agenda.el

@@ -2538,7 +2538,7 @@ For example, if you have a custom agenda command \"p\" and you
 want this command to be accessible only from plain text files,
 use this:
 
-   '((\"p\" ((in-file . \"\\.txt\"))))
+   '((\"p\" ((in-file . \"\\\\.txt\\\\'\"))))
 
 Here are the available contexts definitions:
 
@@ -2556,7 +2556,7 @@ accessible if there is at least one valid check.
 You can also bind a key to another agenda custom command
 depending on contextual rules.
 
-    '((\"p\" \"q\" ((in-file . \"\\.txt\"))))
+    '((\"p\" \"q\" ((in-file . \"\\\\.txt\\\\'\"))))
 
 Here it means: in .txt files, use \"p\" as the key for the
 agenda command otherwise associated with \"q\".  (The command