Math: correct 4 copy-paste errors in header comments (#1316)

Jolt/Math/DVec3.h

@@ -221,10 +221,10 @@ public:
 	/// Subtract two double vectors (component wise)
 	JPH_INLINE DVec3			operator - (DVec3Arg inV2) const;
 
-	/// Add two vectors (component wise)
+	/// Subtract two vectors (component wise)
 	JPH_INLINE DVec3 &			operator -= (Vec3Arg inV2);
 
-	/// Add two double vectors (component wise)
+	/// Subtract two vectors (component wise)
 	JPH_INLINE DVec3 &			operator -= (DVec3Arg inV2);
 
 	/// Divide (component wise)

Jolt/Math/Vec3.h

@@ -189,7 +189,7 @@ public:
 	/// Subtract two float vectors (component wise)
 	JPH_INLINE Vec3				operator - (Vec3Arg inV2) const;
 
-	/// Add two float vectors (component wise)
+	/// Subtract two float vectors (component wise)
 	JPH_INLINE Vec3 &			operator -= (Vec3Arg inV2);
 
 	/// Divide (component wise)

Jolt/Math/Vec4.h

@@ -175,7 +175,7 @@ public:
 	/// Subtract two float vectors (component wise)
 	JPH_INLINE Vec4				operator - (Vec4Arg inV2) const;
 
-	/// Add two float vectors (component wise)
+	/// Subtract two float vectors (component wise)
 	JPH_INLINE Vec4 &			operator -= (Vec4Arg inV2);
 
 	/// Divide (component wise)