Update Base.agda

fixed the error

Author: anshwad10

Cubical/Data/Sigma/Base.agda

@@ -54,5 +54,5 @@ syntax ∃!-syntax A (λ x → B) = ∃![ x ∈ A ] B
 -- Instance
 
 instance
-  Σ-instance : ∀ {ℓ ℓ'} {A : Type ℓ} {B : A → Type ℓ'} ⦃ x : A ⦄ ⦃ B x ⦄ → Σ A B
+  Σ-instance : ∀ {ℓ ℓ'} {A : Type ℓ} {B : A → Type ℓ'} ⦃ x : A ⦄ ⦃ _ : B x ⦄ → Σ A B
   Σ-instance ⦃ x ⦄ ⦃ y ⦄ = (x , y)