Update fir-basics.md

Fix broken link to `ClassId`

Author: NeonMika

docs/fir/fir-basics.md

@@ -78,7 +78,7 @@ generated and written with all possible members explicitly declared, so they are
 The main way to get some declaration inside compiler is [FirSymbolProvider](https://github.com/JetBrains/kotlin/blob/master/compiler/fir/providers/src/org/jetbrains/kotlin/fir/resolve/providers/FirSymbolProvider.kt)
 and [FirScope](https://github.com/JetBrains/kotlin/blob/master/compiler/fir/tree/src/org/jetbrains/kotlin/fir/scopes/FirScope.kt).
 
-_Symbol provider_ is used to lookup for classes by their [ClassId](https://github.com/JetBrains/kotlin/blob/mastercore/compiler.common/src/org/jetbrains/kotlin/name/ClassId.java) and top-level functions and properties by
+_Symbol provider_ is used to lookup for classes by their [ClassId](https://github.com/JetBrains/kotlin/blob/master/core/compiler.common/src/org/jetbrains/kotlin/name/ClassId.kt) and top-level functions and properties by
 their [CallableId](https://github.com/JetBrains/kotlin/blob/master/core/compiler.common/src/org/jetbrains/kotlin/name/CallableId.kt).
 The main symbol provider is a composition of multiple symbol providers, each of them looks up for declaration in specific scopes:
 - In sources of current modules,
@@ -104,4 +104,4 @@ val returnType = functionSymbol.fir.returnTypeRef as FirResolvedTypeRef
 ```
 
 Please don't forget to make sure that you are in the correct compiler phase which can guarantee that required declaration parts are resolved. For example, it's illegal to access resolved return type of some function in `STATUS` stage, because at this point of time implicit return types are not
-resolved. Implicit types are resolved at `IMPLICIT_TYPES_BODY_RESOLVE` phase.
+resolved. Implicit types are resolved at `IMPLICIT_TYPES_BODY_RESOLVE` phase.