Abbreviate "parameter" -> "param" (#3392)

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.

---------

Co-authored-by: Jon Ross-Perkins <jperkins@google.com>

toolchain/check/context.cpp

@@ -818,11 +818,11 @@ class TypeCompleter {
       case SemIR::NameReference::Kind:
       case SemIR::Namespace::Kind:
       case SemIR::NoOp::Kind:
-      case SemIR::Parameter::Kind:
+      case SemIR::Param::Kind:
       case SemIR::RealLiteral::Kind:
       case SemIR::Return::Kind:
       case SemIR::ReturnExpr::Kind:
-      case SemIR::SelfParameter::Kind:
+      case SemIR::SelfParam::Kind:
       case SemIR::SpliceBlock::Kind:
       case SemIR::StringLiteral::Kind:
       case SemIR::StructAccess::Kind: