roffit: Decrease the indent level for .PP

Prior to this change when .PP (regular paragraph) was used after
.IP (indented paragraph) roffit would not change the indent level which
resulted in regular paragraphs having the same indent as the previously
indented paragraph.

Ref: curl/curl#13803 (comment)

Closes #xxxxx

roffit

@@ -502,6 +502,7 @@ sub parsefile {
                 $within_tp=0;
                 showp(@p);
                 @p="";
+                $indentlevel-- if ($indentlevel);
             }
             elsif($keyword =~ /^fi$/i) {
                 # .fi = fill-in, extra space will be ignored and text that is