fix confusing ignore rule: change extension to pdf

Author: madkite

book/02-git-basics/sections/recording-changes.asc

@@ -239,8 +239,8 @@ build/
 # ignore doc/notes.txt, but not doc/server/arch.txt
 doc/*.txt
 
-# ignore all .txt files in the doc/ directory
-doc/**/*.txt
+# ignore all .pdf files in the doc/ directory
+doc/**/*.pdf
 ----
 
 [TIP]