Fix markup. versionchanged automatically adds a period, so remove the extra one.

Doc/lib/libtarfile.tex

@@ -262,7 +262,7 @@ tar archive several times. Each archive member is represented by a
     this limit.  If false, create a GNU tar compatible archive.  It
     will not be \POSIX{} compliant, but can store files without any
     of the above restrictions. 
-    \versionchanged[\var{posix} defaults to false.]{2.4}
+    \versionchanged[\var{posix} defaults to \constant{False}]{2.4}
 \end{memberdesc}
 
 \begin{memberdesc}{dereference}