Fix typo

crates/ra_hir_def/src/body.rs

@@ -27,7 +27,7 @@
     AsMacroCall, DefWithBodyId, HasModule, Lookup, ModuleId,
 };
 
-/// A subser of Exander that only deals with cfg attributes. We only need it to
+/// A subset of Exander that only deals with cfg attributes. We only need it to
 /// avoid cyclic queries in crate def map during enum processing.
 pub(crate) struct CfgExpander {
     cfg_options: CfgOptions,