Use abbreviation "expr" instead of "expression" (#3375)

Part of switching to the [abbreviations we've decided to
use](https://docs.google.com/document/d/1RRYMm42osyqhI2LyjrjockYCutQ5dOf8Abu50kTrkX0/edit?resourcekey=0-kHyqOESbOHmzZphUbtLrTw#heading=h.pph7i5m5un7q).

I will rename files in a follow-up PR.

toolchain/parse/handle_function.cpp

@@ -28,7 +28,7 @@ auto HandleFunctionAfterParameters(Context& context) -> void {
   if (context.PositionIs(Lex::TokenKind::MinusGreater)) {
     context.PushState(State::FunctionReturnTypeFinish);
     ++context.position();
-    context.PushStateForExpression(PrecedenceGroup::ForType());
+    context.PushStateForExpr(PrecedenceGroup::ForType());
   }
 }