fix(fstring): use x/X for hexadecimal instead of h/H

examples/basic/23_format_spec.peb

@@ -1,5 +1,5 @@
 let a = 42
-print(f"int: {a:d}; hex: {a:h}; HEX: {a:H}; oct: {a:o}; bin: {a:b}")
+print(f"int: {a:d}; hex: {a:x}; HEX: {a:X}; oct: {a:o}; bin: {a:b}")
 let b = 1234567890
 print(f"{b:,}")
 print(f"{b:_}")

src/core/format_spec/lexer.py

@@ -95,7 +95,7 @@ class FormatSpecLexer:
                 self.add_token(TokenType.T_DEC)
             case "o":
                 self.add_token(TokenType.T_OCT)
-            case "h" | "H":
+            case "x" | "X":
                 self.add_token(TokenType.T_HEX)
             case "e":
                 self.add_token(TokenType.T_SCI)