Fix bug with ` within literal blocks

These would previously be eaten if not escaped.

scdoc.1.scd

@@ -85,9 +85,9 @@ to be treated as a literal and copied directly to the output. You may also make
 blocks of literal syntax like so:
 
 ```
-\`\`\`
+\```
 _This formatting_ will *not* be interpreted by scdoc.
-\`\`\`
+\```
 ```
 
 These blocks will be indented one level. Note that literal text is shown

src/main.c

@@ -234,7 +234,10 @@ static void parse_literal(struct parser *p, int *indent) {
 				return;
 			}
 		} else {
-			stops = 0;
+			while (stops != 0) {
+				fputc('`', p->output);
+				--stops;
+			}
 			switch (ch) {
 			case '.':
 				fprintf(p->output, "\\&.");