fix: correct typo in invalid reassignment error (leanprover#5080)

Corrects a small typo in the error message for when a user attempts to
mutate something which cannot be mutated.

src/Lean/Elab/Do.lean

@@ -1264,7 +1264,7 @@ def withNewMutableVars {α} (newVars : Array Var) (mutable : Bool) (x : M α) :
 
 def checkReassignable (xs : Array Var) : M Unit := do
   let throwInvalidReassignment (x : Name) : M Unit :=
-    throwError "`{x.simpMacroScopes}` cannot be mutated, only variables declared using `let mut` can be mutated. If you did not intent to mutate but define `{x.simpMacroScopes}`, consider using `let {x.simpMacroScopes}` instead"
+    throwError "`{x.simpMacroScopes}` cannot be mutated, only variables declared using `let mut` can be mutated. If you did not intend to mutate but define `{x.simpMacroScopes}`, consider using `let {x.simpMacroScopes}` instead"
   let ctx ← read
   for x in xs do
     unless ctx.mutableVars.contains x.getId do

tests/lean/doNotation1.lean.expected.out

@@ -1,6 +1,6 @@
-doNotation1.lean:4:0-4:6: error: `y` cannot be mutated, only variables declared using `let mut` can be mutated. If you did not intent to mutate but define `y`, consider using `let y` instead
-doNotation1.lean:8:2-8:18: error: `y` cannot be mutated, only variables declared using `let mut` can be mutated. If you did not intent to mutate but define `y`, consider using `let y` instead
-doNotation1.lean:12:2-12:17: error: `p` cannot be mutated, only variables declared using `let mut` can be mutated. If you did not intent to mutate but define `p`, consider using `let p` instead
+doNotation1.lean:4:0-4:6: error: `y` cannot be mutated, only variables declared using `let mut` can be mutated. If you did not intend to mutate but define `y`, consider using `let y` instead
+doNotation1.lean:8:2-8:18: error: `y` cannot be mutated, only variables declared using `let mut` can be mutated. If you did not intend to mutate but define `y`, consider using `let y` instead
+doNotation1.lean:12:2-12:17: error: `p` cannot be mutated, only variables declared using `let mut` can be mutated. If you did not intend to mutate but define `p`, consider using `let p` instead
 doNotation1.lean:20:7-20:22: error: invalid reassignment, value has type
   Vector Nat (n + 1) : Type
 but is expected to have type