doc: escape braces in AsciiDoc

This commit fixes a bug where AsciiDoc would drop any line containing a
'{foo}' because it interpreted it as an undefined attribute reference:

> Simple attribute references take the form {<name>}. If the attribute name
> is defined its text value is substituted otherwise the line containing the
> reference is dropped from the output.

See: https://www.methods.co.nz/asciidoc/chunked/ch30.html

We fix this by simply replacing all occurrences of '{' and '}' with
their escaped forms: '&#123;' and '&#125;'.

Fixes #1101

build.rs

@@ -168,7 +168,12 @@ fn formatted_arg(arg: &RGArg) -> io::Result<String> {
 }
 
 fn formatted_doc_txt(arg: &RGArg) -> io::Result<String> {
-    let paragraphs: Vec<&str> = arg.doc_long.split("\n\n").collect();
+    let paragraphs: Vec<String> = arg.doc_long
+        .replace("{", "&#123;")
+        .replace("}", r"&#125;")
+        .split("\n\n")
+        .map(|s| s.to_string())
+        .collect();
     if paragraphs.is_empty() {
         return Err(ioerr(format!("missing docs for --{}", arg.name)));
     }