internal: Make COMPLETION_MARKER more explicitly r-a

If a user ever sees the completion marker, it's confusing to see text
about IntelliJ. Use a string that's more explicitly about completion
for rust-analyzer.

src/tools/rust-analyzer/crates/ide-completion/src/context.rs

@@ -26,7 +26,7 @@
     CompletionConfig,
 };
 
-const COMPLETION_MARKER: &str = "intellijRulezz";
+const COMPLETION_MARKER: &str = "raCompletionMarker";
 
 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
 pub(crate) enum PatternRefutability {