Improve gitignore file

One file was listed under a wrong comment.

Author: petere

doc/src/sgml/.gitignore

@@ -7,7 +7,6 @@
 /man-stamp
 # Other popular build targets
 /INSTALL
-/INSTALL.xml
 /postgres-US.pdf
 /postgres-A4.pdf
 /postgres.html
@@ -22,6 +21,7 @@
 # Assorted byproducts from building the above
 /postgres.xml
 /INSTALL.html
+/INSTALL.xml
 /postgres-US.aux
 /postgres-US.log
 /postgres-US.out