docs: Remove useless p:first-line CSS rule

According to MDN[1], 'margin-left' and similar CSS properties,
including 'margin-right', cannot be applied to the '::first-line'
pseudo-element, so this rule will never have any effect and can
be safely removed.

[1] https://developer.mozilla.org/en/docs/Web/CSS/::first-line

docs/generic.css

@@ -7,10 +7,6 @@ body {
   background: #ffffff;
 }
 
-p:first-line {
-  margin-right: 1em;
-}
-
 div.body p:first-letter {
   font-size: 1.2em;
   font-weight: bold;