documentation: consistent abbreviation for URL (#3684)

I noticed `fetch`'s documentation used "URL" so wanted to do so here as
well.

crates/nu_plugin_post/src/nu/mod.rs

@@ -9,7 +9,7 @@ use crate::Post;
 impl Plugin for Post {
     fn config(&mut self) -> Result<Signature, ShellError> {
         Ok(Signature::build("post")
-            .desc("Post content to a url and retrieve data as a table if possible.")
+            .desc("Post content to a URL and retrieve data as a table if possible.")
             .required("path", SyntaxShape::Any, "the URL to post to")
             .required("body", SyntaxShape::Any, "the contents of the post body")
             .named(

docs/commands/post.md

@@ -1,5 +1,5 @@
 # post
-Post content to a url and retrieve data as a table if possible.
+Post content to a URL and retrieve data as a table if possible.
 
 ## Usage
 ```shell