Detect URL properly, such that correct API call is made

This fixes issue #2. Whenever a text contains an URL anywhere, the
kagi-url-p function would return positive and the text is submitted as
an URL to the Kagi Summarizer API. Now this is only done when the
string is a URL only, otherwise the text summarizer is invoked.

kagi.el

@@ -337,7 +337,7 @@ Returns a formatted string to be displayed by the shell."
 
 (defun kagi--url-p (s)
   "Non-nil if string S is a URL."
-  (string-match-p (rx (seq "http" (? "s") "://")) s))
+  (string-match-p (rx (seq bos "http" (? "s") "://" (+ (not space)) eos)) s))
 
 ;;;###autoload
 (defun kagi-summarize (text-or-url)