Correct incorrect "it's" to "its".

Doc/lib/libcgitb.tex

@@ -13,7 +13,7 @@
 \index{tracebacks!in CGI scripts}
 
 The \module{cgitb} module provides a special exception handler for Python
-scripts.  (It's name is a bit misleading.  It was originally designed to
+scripts.  (Its name is a bit misleading.  It was originally designed to
 display extensive traceback information in HTML for CGI scripts.  It was
 later generalized to also display this information in plain text.)  After
 this module is activated, if an uncaught exception occurs, a detailed,