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/context.cpp

@@ -144,8 +144,8 @@ auto Context::ConsumeIfPatternKeyword(Lex::TokenKind keyword_token,
     -> void {
   if (auto token = ConsumeIf(keyword_token)) {
     PushState(Context::StateStackEntry(
-        keyword_state, PrecedenceGroup::ForTopLevelExpression(),
-        PrecedenceGroup::ForTopLevelExpression(), *token, subtree_start));
+        keyword_state, PrecedenceGroup::ForTopLevelExpr(),
+        PrecedenceGroup::ForTopLevelExpr(), *token, subtree_start));
   }
 }